The use of equations is widespread in our lives. They are used in many calculations, construction of structures and even sports. Equations have been used by man since ancient times and since then their use has only increased. In mathematics, equations of higher degrees with integer coefficients are quite common. To solve this kind of equation, you need:

Determine the rational roots of the equation;

Factor out the polynomial that is on the left side of the equation;

Find the roots of the equation.

Suppose we are given an equation the following kind:

Let's find all its real roots. Multiply the left and right sides of the equation by \

Let's change the variables \

Thus, we have obtained a reduced equation of the fourth degree, which is solved according to the standard algorithm: we check the divisors, carry out the division, and as a result we find out that the equation has two real roots \ and two complex ones. We get the following answer to our equation of the fourth degree:

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Methods for solving equations: n n n Replacement of the equation h(f(x)) = h(g(x)) by the equation f(x) = g(x) Factorization. Introduction of a new variable. Functional - graphical method. Root selection. Application of Vieta formulas.

Replacing the equation h(f(x)) = h(g(x)) by the equation f(x) = g(x). The method can be applied only when y = h(x) is a monotonic function that takes each of its values ​​once. If the function is nonmonotone, then the loss of roots is possible.

Solve the equation (3 x + 2)²³ = (5 x - 9)²³ y = x ²³ increasing function, so from the equation (3 x + 2)²³ = (5 x - 9)²³ you can go to the equation 3 x + 2 \u003d 5 x - 9, from where we find x \u003d 5.5. Answer: 5.5.

Factorization. The equation f(x)g(x)h(x) = 0 can be replaced by the set of equations f(x) = 0; g(x) = 0; h(x) = 0. Having solved the equations of this set, you need to take those roots that belong to the domain of definition of the original equation, and discard the rest as extraneous.

Solve the equation x³ - 7 x + 6 = 0 Representing the term 7 x as x + 6 x, we get sequentially: x³ - x - 6 x + 6 = 0 x(x² - 1) - 6(x - 1) = 0 x (x - 1)(x + 1) - 6(x - 1) = 0 (x - 1)(x² + x - 6) = 0 Now the problem is reduced to solving a set of equations x - 1 = 0; x² + x - 6 = 0. Answer: 1, 2, - 3.

Introduction of a new variable. If the equation y(x) = 0 can be transformed to the form p(g(x)) = 0, then you need to introduce a new variable u = g(x), solve the equation p(u) = 0, and then solve the set of equations g( x) = u 1; g(x) = u2; … ; g(x) = un , where u 1, u 2, … , un are the roots of the equation p(u) = 0.

Solve the equation A feature of this equation is the equality of the coefficients of its left side, equidistant from its ends. Such equations are called reciprocal. Since 0 is not the root of this equation, dividing by x² gives

Let's introduce a new variable Then We get a quadratic equation So the root y 1 = - 1 can be ignored. We get the Answer: 2, 0, 5.

Solve the equation 6(x² - 4)² + 5(x² - 4)(x² - 7 x +12) + (x² - 7 x + 12)² = 0 This equation can be solved as homogeneous. Divide both sides of the equation by (x² - 7 x +12)² (it is clear that x values ​​such that x² - 7 x +12=0 are not solutions). Now let's denote We Have From Here Answer:

Functional - graphical method. If one of the functions y \u003d f (x), y \u003d g (x) increases, and the other decreases, then the equation f (x) \u003d g (x) either has no roots or has one root.

Solve the equation It is quite obvious that x = 2 is the root of the equation. Let us prove that this is the only root. We transform the equation to the form We notice that the function is increasing, and the function is decreasing. So the equation has only one root. Answer: 2.

Selection of roots n n n Theorem 1: If an integer m is a root of a polynomial with integer coefficients, then the constant term of the polynomial is divisible by m. Theorem 2: The reduced polynomial with integer coefficients has no fractional roots. Theorem 3: – equation with integer Let coefficients. If the number and fraction where p and q are integers is irreducible, is the root of the equation, then p is the divisor of the free term an, and q is the divisor of the coefficient at the highest term a 0.

Bezout's theorem. The remainder when dividing any polynomial by a binomial (x - a) is equal to the value of the divisible polynomial at x = a. Consequences of Bezout's theorem n n n n The difference of identical powers of two numbers is divisible without remainder by the difference of the same numbers; The difference of identical even powers of two numbers is divisible without remainder both by the difference of these numbers and by their sum; The difference of identical odd powers of two numbers is not divisible by the sum of these numbers; The sum of equal powers of two non-numbers is divisible by the difference of these numbers; The sum of identical odd powers of two numbers is divisible without a remainder by the sum of these numbers; The sum of identical even powers of two numbers is not divisible either by the difference of these numbers or by their sum; The polynomial is divisible by the binomial (x - a) if and only if the number a is the root of this polynomial; The number of distinct roots of a non-zero polynomial is no more than its degree.

Solve the equation x³ - 5 x² - x + 21 = 0 The polynomial x³ - 5 x² - x + 21 has integer coefficients. By Theorem 1, its integer roots, if any, are among the divisors of the free term: ± 1, ± 3, ± 7, ± 21. By checking, we make sure that the number 3 is a root. By a corollary of Bezout's theorem, the polynomial is divisible by (x – 3). Thus, x³ - 5 x² - x + 21 \u003d (x - 3) (x² - 2 x - 7). Answer:

Solve the equation 2 x³ - 5 x² - x + 1 = 0 According to Theorem 1, only numbers ± 1 can be integer roots of the equation. Checking shows that these numbers are not roots. Since the equation is not reduced, it can have fractional rational roots. Let's find them. To do this, multiply both sides of the equation by 4: 8 x³ - 20 x² - 4 x + 4 = 0 By substituting 2 x = t, we get t³ - 5 t² - 2 t + 4 = 0. By Terem 2, all rational roots of this reduced equation must be whole. They can be found among the divisors of the constant term: ± 1, ± 2, ± 4. In this case, t \u003d - 1 is suitable. Therefore, the polynomial 2 x³ - 5 x² - x + 1 is divisible by (x + 0, 5 ): 2 x³ - 5 x² - x + 1 \u003d (x + 0, 5) (2 x² - 6 x + 2) Solving the quadratic equation 2 x² - 6 x + 2 \u003d 0, we find the remaining roots: Answer:

Solve the equation 6 x³ + x² - 11 x - 6 = 0 According to Theorem 3, the rational roots of this equation should be sought among the numbers. Substituting them one by one into the equation, we find that they satisfy the equation. They exhaust all the roots of the equation. Answer:

Find the sum of squares of the roots of the equation x³ + 3 x² - 7 x +1 = 0 By the Vieta theorem Note that whence

Specify the method by which each of these equations can be solved. Solve Equations #1, 4, 15, 17.

Answers and instructions: 1. Introduction of a new variable. 2. Functional - graphical method. 3. Replacing the equation h(f(x)) = h(g(x)) by the equation f(x) = g(x). 4. Factorization. 5. Selection of roots. 6 Functionally - graphical method. 7. Application of Vieta formulas. 8. Selection of roots. 9. Replacing the equation h(f(x)) = h(g(x)) by the equation f(x) = g(x). 10. Introduction of a new variable. 11. Factorization. 12. Introduction of a new variable. 13. Selection of roots. 14. Application of Vieta formulas. 15. Functional - graphical method. 16. Factorization. 17. Introduction of a new variable. 18. Factorization.

1. Instruction. Write the equation as 4(x²+17 x+60)(x+16 x+60)=3 x², Divide both sides by x². Enter variable Answer: x 1 = - 8; x 2 \u003d - 7, 5. 4. Indication. Add 6 y and - 6 y to the left side of the equation and write it as (y³ - 2 y²) + (- 3 y² + 6 y) + (- 8 y + 16) = (y - 2)(y² - 3 y - eight). Answer:

14. Instruction. According to Vieta's theorem Since - are integers, then only the numbers - 1, - 2, - 3 can be the roots of the equation. Answer: 15. Answer: - 1. 17. Indication. Divide both sides of the equation by x² and write it as Enter a variable Answer: 1; fifteen; 2; 3.

Bibliography. n n n Kolmogorov A. N. “Algebra and the Beginnings of Analysis, 10 – 11” (M.: Prosveshchenie, 2003). Bashmakov M. I. "Algebra and the beginning of analysis, 10 - 11" (M.: Education, 1993). Mordkovich A. G. "Algebra and the beginning of analysis, 10 - 11" (M.: Mnemozina, 2003). Alimov Sh. A., Kolyagin Yu. M. et al. “Algebra and the Beginnings of Analysis, 10 – 11” (M.: Prosveshchenie, 2000). Galitsky M. L., Goldman A. M., Zvavich L. I. "Collection of problems in algebra, 8 - 9" (M .: Education, 1997). Karp A.P. "Collection of problems in algebra and the beginnings of analysis, 10 - 11" (M .: Education, 1999). Sharygin I. F. "Optional course in mathematics, problem solving, 10" (M.: Education. 1989). Skopets Z. A. “Additional chapters in the course of mathematics, 10” (M .: Education, 1974). Litinsky G.I. "Lessons in Mathematics" (Moscow: Aslan, 1994). Muravin G. K. "Equations, inequalities and their systems" (Mathematics, supplement to the newspaper "First of September", No. 2, 3, 2003). Kolyagin Yu. M. "Polynomials and equations of higher degrees" (Mathematics, supplement to the newspaper "First of September", No. 3, 2005).

Trifanova Marina Anatolievna
Mathematics teacher, Gymnasium No. 48 (multiprofile)

The triune purpose of the lesson:

Educational:
systematization and generalization of knowledge on solving equations of higher degrees.
Developing:
to promote the development of logical thinking, the ability to work independently, the skills of mutual control and self-control, the ability to speak and listen.
Nurturing:
development of the habit of constant employment, education of responsiveness, hard work, accuracy.

Lesson type:

a lesson in the complex application of knowledge, skills and abilities.

Lesson Form:

airing, physical minute, various forms of work.

Equipment:

reference notes, task cards, lesson monitoring matrix.

DURING THE CLASSES

I. Organizational moment

1. Communicate the purpose of the lesson to students.
2. Checking homework (Appendix 1). Work with the basic abstract (Appendix 2).

Equations and answers for each are written on the board. Students check the answers and give a brief analysis of the solution to each equation or answer the teacher's questions (frontal survey). Self-control - students give themselves grades and hand over notebooks to the teacher to check for correction of grades or their approval. Grades school written on the blackboard:

“5+” - 6 equations;
“5” - 5 equations;
“4” - 4 equations;
“3” - 3 equations.

Teacher questions for homework:

1 equation

1. What is the change of variables in the equation?
2. What equation is obtained after the change of variables?

2 equation

1. What polynomial divided both sides of the equation?
2. What substitution of variables was obtained?

3 equation

1. What polynomials need to be multiplied to simplify the solution of this equation?

4 equation

1. Name the function f(x).
2. How were the other roots found?

5 equation

1. How many intervals were obtained to solve the equation?

6 equation

1. How could this equation be solved?
2. Which solution is more rational?

II. Group work is the main part of the lesson.

The class is divided into 4 groups. Each group is given a card with theoretical and practical (Appendix 3) questions: "Disassemble the proposed method for solving the equation and explain it using this example."

1. Group work 15 minutes.
2. Examples are written on the board (the board is divided into 4 parts).
3. The group report takes 2-3 minutes.
4. The teacher corrects the reports of the groups and helps in case of difficulty.

Group work continues on cards No. 5 - 8. For each equation, 5 minutes are given for discussion in the group. Then the blackboard has a report on this equation - a brief analysis of the solution. The equation may not be completely solved - it is being finalized at home, but the entire sequence of its solution in the class is discussed.

III. Independent work. Annex 4.

1. Each student receives an individual assignment.
2. The job takes 20 minutes.
3. 5 minutes before the end of the lesson, the teacher gives open answers for each equation.
4. Students change notebooks in a circle and check the answers with a friend. Giving ratings.
5. Notebooks are handed over to the teacher for checking and correcting grades.

IV. Summary of the lesson.

Homework.

Complete the solution of incomplete equations. Prepare for the control cut.

Grading.

Basic goals:

1. To consolidate the concept of an integer rational equation of the th degree.
2. Formulate the main methods for solving equations of higher degrees (n > 3).
3. To teach the basic methods for solving equations of higher degrees.
4. To teach by the form of the equation to determine the most effective way to solve it.

Forms, methods and pedagogical techniques that are used by the teacher in the classroom:

• Lecture-seminar training system (lectures - explanation of new material, seminars - problem solving).
• Information and communication technologies (frontal survey, oral work with the class).
• Differentiated training, group and individual forms.
• The use of the research method in teaching, aimed at developing the mathematical apparatus and mental abilities of each individual student.
• Printed material - an individual summary of the lesson (basic concepts, formulas, statements, lecture material is compressed in the form of diagrams or tables).

Lesson plan:

1. Organizing time.
   The purpose of the stage: to include students in learning activities, to determine the content of the lesson.
2. Updating students' knowledge.
   The purpose of the stage: to update the knowledge of students on previously studied related topics
3. Learning a new topic (lecture). The purpose of the stage: to formulate the main methods for solving equations of higher degrees (n > 3)
4. Summarizing.
   The purpose of the stage: to once again highlight the key points in the material studied in the lesson.
5. Homework.
   The purpose of the stage: to formulate homework for students.

Lesson summary

1. Organizational moment.

The wording of the topic of the lesson: “Equations of higher degrees. Methods for their solution”.

2. Actualization of students' knowledge.

Theoretical survey - conversation. Repetition of some previously studied information from the theory. Students formulate basic definitions and give statements of necessary theorems. Examples are given, demonstrating the level of previously acquired knowledge.

• The concept of an equation with one variable.
• The concept of the root of the equation, the solution of the equation.
• The concept of a linear equation with one variable, the concept of a quadratic equation with one variable.
• The concept of equivalence of equations, equation-consequences (the concept of extraneous roots), transition not by consequence (the case of loss of roots).
• The concept of a whole rational expression with one variable.
• The concept of an entire rational equation n th degree. The standard form of an entire rational equation. Reduced whole rational equation.
• Transition to a set of equations of lower degrees by factoring the original equation.
• The concept of a polynomial n th degree from x. Bezout's theorem. Consequences from Bezout's theorem. Root theorems ( Z-roots and Q-roots) of an entire rational equation with integer coefficients (reduced and nonreduced, respectively).
• Horner's scheme.

3. Learning a new topic.

We will consider the whole rational equation n th power of the standard form with one unknown variable x:Pn(x)= 0 , where P n (x) = a n x n + a n-1 x n-1 + a 1 x + a 0– polynomial n th degree from x, a n ≠ 0 . If a a n = 1 then such an equation is called a reduced whole rational equation n th degree. Let us consider such equations for different values n and list the main methods of their solution.

n= 1 is a linear equation.

n= 2 is a quadratic equation. Discriminant formula. Formula for calculating roots. Vieta's theorem. Selection of a full square.

n= 3 is a cubic equation.

grouping method.

Example: x 3 – 4x 2 – x+ 4 = 0 (x - 4) (x 2– 1) = 0 x 1 = 4 , x2 = 1,x 3 = -1.

Reciprocal cubic equation of the form ax 3 + bx 2 + bx + a= 0. We solve by combining terms with the same coefficients.

Example: x 3 – 5x 2 – 5x + 1 = 0 (x + 1)(x 2 – 6x + 1) = 0 x 1 = -1, x 2 = 3 + 2, x 3 = 3 – 2.

Selection of Z-roots based on the theorem. Horner's scheme. When applying this method, it is necessary to emphasize that the enumeration in this case is finite, and we select the roots according to a certain algorithm in accordance with the theorem on Z-roots of the reduced whole rational equation with integer coefficients.

Example: x 3 – 9x 2 + 23x– 15 = 0. The equation is reduced. We write out the divisors of the free term ( + 1; + 3; + 5; + fifteen). Let's apply Horner's scheme:

	x 3	x 2	x 1	x 0	conclusion
	1	-9	23	-15
1	1	1 x 1 - 9 = -8	1 x (-8) + 23 = 15	1 x 15 - 15 = 0	1 - root
	x 2	x 1	x 0

We get ( x – 1)(x 2 – 8x + 15) = 0 x 1 = 1, x 2 = 3, x 3 = 5.

Equation with integer coefficients. Selection of Q-roots based on the theorem. Horner's scheme. When applying this method, it is necessary to emphasize that the enumeration in this case is finite and we select the roots according to a certain algorithm in accordance with the theorem on Q-roots of an unreduced whole rational equation with integer coefficients.

Example: 9 x 3 + 27x 2 – x– 3 = 0. The equation is not reduced. We write out the divisors of the free term ( + 1; + 3). Let us write out the divisors of the coefficient at the highest power of the unknown. ( + 1; + 3; + 9) Therefore, we will look for roots among the values ​​( + 1; + ; + ; + 3). Let's apply Horner's scheme:

	x 3	x 2	x 1	x 0	conclusion
	9	27	-1	-3
1	9	1 x 9 + 27 = 36	1 x 36 - 1 = 35	1 x 35 - 3 = 32 ≠ 0	1 is not a root
-1	9	-1 x 9 + 27 = 18	-1 x 18 - 1 = -19	-1 x (-19) - 3 = 16 ≠ 0	-1 is not a root
	9	x9 + 27 = 30	x 30 - 1 = 9	x 9 - 3 = 0	root
	x 2	x 1	x 0

We get ( x – )(9x 2 + 30x + 9) = 0 x 1 = , x 2 = - , x 3 = -3.

For the convenience of calculation when choosing Q -roots it can be convenient to make a change of variable, go to the above equation and adjust Z -roots.

• If the intercept is 1

.

• If it is possible to use the substitution of the form y=kx

.

Formula Cardano. There is a universal method for solving cubic equations - this is the Cardano formula. This formula is associated with the names of the Italian mathematicians Gerolamo Cardano (1501–1576), Nicolo Tartaglia (1500–1557), Scipio del Ferro (1465–1526). This formula lies outside the scope of our course.

n= 4 is an equation of the fourth degree.

grouping method.

Example: x 4 + 2x 3 + 5x 2 + 4x – 12 = 0 (x 4 + 2x 3) + (5x 2 + 10x) – (6x + 12) = 0 (x + 2)(x 3 + 5x- 6) = 0 (x + 2)(x– 1)(x 2 + x + 6) = 0 x 1 = -2, x 2 = 1.

Variable replacement method.

• Biquadratic equation of the form ax 4 + bx 2+s = 0 .

Example: x 4 + 5x 2 - 36 = 0. Substitution y = x 2. From here y 1 = 4, y 2 = -9. So x 1,2 = + 2 .

• Reciprocal equation of the fourth degree of the form ax 4 + bx 3+c x 2 + bx + a = 0.

We solve by combining terms with the same coefficients by replacing the form

• ax 4 + bx 3 + cx 2 – bx + a = 0.

• Generalized backward equation of the fourth degree of the form ax 4 + bx 3 + cx 2 + kbx + k2 a = 0.

• General replacement. Some standard substitutions.

Example 3 . General view replacement(follows from the form of a particular equation).

n = 3.

Equation with integer coefficients. Selection of Q-roots n = 3.

General formula. There is a universal method for solving equations of the fourth degree. This formula is associated with the name of Ludovico Ferrari (1522-1565). This formula lies outside the scope of our course.

n > 5 - equations of the fifth and higher degrees.

Equation with integer coefficients. Selection of Z-roots based on the theorem. Horner's scheme. The algorithm is similar to the one discussed above for n = 3.

Equation with integer coefficients. Selection of Q-roots based on the theorem. Horner's scheme. The algorithm is similar to the one discussed above for n = 3.

Symmetric equations. Any reciprocal equation of odd degree has a root x= -1 and after decomposing it into factors, we get that one factor has the form ( x+ 1), and the second factor is a reciprocal equation of even degree (its degree is one less than the degree of the original equation). Any reciprocal equation of even degree together with a root of the form x = φ also contains the root of the form . Using these statements, we solve the problem by lowering the degree of the equation under study.

Variable replacement method. Use of homogeneity.

There is no general formula for solving entire fifth-degree equations (this was shown by the Italian mathematician Paolo Ruffini (1765–1822) and the Norwegian mathematician Nils Henrik Abel (1802–1829)) and higher powers (this was shown by the French mathematician Evariste Galois (1811–1832) )).

• Recall again that in practice it is possible to use combinations the methods listed above. It is convenient to pass to a set of equations of lower degrees by factorization of the original equation.
• Outside the scope of our today's discussion, there are widely used in practice graphic methods solving equations and approximate solution methods equations of higher degrees.
• There are situations when the equation does not have R-roots.
Then the solution comes down to showing that the equation has no roots. To prove this, we analyze the behavior of the considered functions on intervals of monotonicity. Example: Equation x 8 – x 3 + 1 = 0 has no roots.
• Using the monotonicity property of functions
. There are situations when the use of various properties of functions allows us to simplify the task.
Example 1: Equation x 5 + 3x– 4 = 0 has one root x= 1. By the property of monotonicity of the analyzed functions, there are no other roots.
Example 2: Equation x 4 + (x– 1) 4 = 97 has roots x 1 = -2 and x 2 = 3. Having analyzed the behavior of the corresponding functions on the intervals of monotonicity, we conclude that there are no other roots.

4. Summing up.

Summary: Now we have mastered the basic methods for solving various equations of higher degrees (for n > 3). Our task is to learn how to effectively use the above algorithms. Depending on the type of equation, we will have to learn how to determine which solution method is the most effective in this case, as well as correctly apply the chosen method.

5. Homework.

: item 7, pp. 164–174, nos. 33–36, 39–44, 46,47.

: №№ 9.1–9.4, 9.6–9.8, 9.12, 9.14–9.16, 9.24–9.27.

Possible topics of reports or abstracts on this topic:

• Formula Cardano
• Graphical method for solving equations. Solution examples.
• Methods for approximate solution of equations.

Analysis of the assimilation of the material and students' interest in the topic:

Experience shows that the interest of students in the first place is the possibility of selecting Z-roots and Q-roots of equations using a fairly simple algorithm using Horner's scheme. Students are also interested in various standard types of variable substitution, which can significantly simplify the type of problem. Graphical methods of solution are usually of particular interest. In this case, you can additionally parse the tasks into a graphical method for solving equations; discuss the general view of the graph for a polynomial of 3, 4, 5 degrees; analyze how the number of roots of equations of 3, 4, 5 degrees is related to the type of the corresponding graph. Below is a list of books where you can find additional information on this topic.

Bibliography:

1. Vilenkin N.Ya. etc. “Algebra. A textbook for students in grades 9 with an in-depth study of mathematics ”- M., Education, 2007 - 367 p.
2. Vilenkin N.Ya., Shibasov L.P., Shibasova Z.F.“Behind the pages of a mathematics textbook. Arithmetic. Algebra. Grades 10-11” – M., Enlightenment, 2008 – 192 p.
3. Vygodsky M.Ya."Handbook of mathematics" - M., AST, 2010 - 1055 p.
4. Galitsky M.L.“Collection of problems in algebra. Textbook for grades 8-9 with in-depth study of mathematics ”- M., Education, 2008 - 301 p.
5. Zvavich L.I. et al. “Algebra and the Beginnings of Analysis. 8–11 cells A manual for schools and classes with in-depth study of mathematics ”- M., Drofa, 1999 - 352 p.
6. Zvavich L.I., Averyanov D.I., Pigarev B.P., Trushanina T.N.“Assignments in mathematics to prepare for a written exam in grade 9” - M., Education, 2007 - 112 p.
7. Ivanov A.A., Ivanov A.P.“Thematic tests for the systematization of knowledge in mathematics” part 1 - M., Fizmatkniga, 2006 - 176 p.
8. Ivanov A.A., Ivanov A.P.“Thematic tests for the systematization of knowledge in mathematics” part 2 - M., Fizmatkniga, 2006 - 176 p.
9. Ivanov A.P.“Tests and tests in mathematics. Tutorial". - M., Fizmatkniga, 2008 - 304 p.
10. Leibson K.L.“Collection of practical tasks in mathematics. Part 2–9 class” – M., MTsNMO, 2009 – 184 p.
11. Makarychev Yu.N., Mindyuk N.G."Algebra. Additional chapters for the 9th grade school textbook. Textbook for students of schools and classes with in-depth study of mathematics.” - M., Education, 2006 - 224 p.
12. Mordkovich A.G."Algebra. In-depth study. 8th grade. Textbook” – M., Mnemosyne, 2006 – 296 p.
13. Savin A.P.“Encyclopedic Dictionary of a Young Mathematician” - M., Pedagogy, 1985 - 352 p.
14. Survillo G.S., Simonov A.S.“Didactic materials on algebra for grade 9 with in-depth study of mathematics” - M., Education, 2006 - 95 p.
15. Chulkov P.V.“Equations and inequalities in the school course of mathematics. Lectures 1–4” – M., First of September, 2006 – 88 p.
16. Chulkov P.V.“Equations and inequalities in the school course of mathematics. Lectures 5–8” – M., First of September, 2009 – 84 p.

In general, an equation that has a degree higher than 4 cannot be solved in radicals. But sometimes we can still find the roots of the polynomial on the left in the equation of the highest degree, if we represent it as a product of polynomials in a degree of no more than 4. The solution of such equations is based on the decomposition of the polynomial into factors, so we advise you to review this topic before studying this article.

Most often, one has to deal with equations of higher degrees with integer coefficients. In these cases, we can try to find rational roots, and then factor the polynomial so that we can then convert it to an equation of a lower degree, which will be easy to solve. In the framework of this material, we will consider just such examples.

Yandex.RTB R-A-339285-1

Higher degree equations with integer coefficients

All equations of the form a n x n + a n - 1 x n - 1 + . . . + a 1 x + a 0 = 0 , we can reduce to an equation of the same degree by multiplying both sides by a n n - 1 and changing the variable like y = a n x:

a n x n + a n - 1 x n - 1 + . . . + a 1 x + a 0 = 0 a n n x n + a n - 1 a n n - 1 x n - 1 + ... + a 1 (a n) n - 1 x + a 0 (a n) n - 1 = 0 y = a n x ⇒ y n + b n - 1 y n - 1 + … + b 1 y + b 0 = 0

The resulting coefficients will also be integers. Thus, we will need to solve the reduced equation of the nth degree with integer coefficients, which has the form x n + a n x n - 1 + ... + a 1 x + a 0 = 0.

We calculate the integer roots of the equation. If the equation has integer roots, you need to look for them among the divisors of the free term a 0. Let's write them down and substitute them into the original equality one by one, checking the result. Once we have obtained an identity and found one of the roots of the equation, we can write it in the form x - x 1 · P n - 1 (x) = 0 . Here x 1 is the root of the equation, and P n - 1 (x) is the quotient of x n + a n x n - 1 + ... + a 1 x + a 0 divided by x - x 1 .

Substitute the remaining divisors in P n - 1 (x) = 0 , starting with x 1 , since the roots can be repeated. After obtaining the identity, the root x 2 is considered found, and the equation can be written as (x - x 1) (x - x 2) P n - 2 (x) \u003d 0. Here P n - 2 (x) will be quotient from dividing P n - 1 (x) by x - x 2 .

We continue to sort through the divisors. Find all integer roots and denote their number as m. After that, the original equation can be represented as x - x 1 x - x 2 · … · x - x m · P n - m (x) = 0 . Here P n - m (x) is a polynomial of n - m -th degree. For calculation it is convenient to use Horner's scheme.

If our original equation has integer coefficients, we cannot end up with fractional roots.

As a result, we got the equation P n - m (x) = 0, the roots of which can be found in any convenient way. They can be irrational or complex.

Let us show on a specific example how such a solution scheme is applied.

Example 1

Condition: find the solution of the equation x 4 + x 3 + 2 x 2 - x - 3 = 0 .

Decision

Let's start with finding integer roots.

We have an intercept equal to minus three. It has divisors equal to 1 , - 1 , 3 and - 3 . Let's substitute them into the original equation and see which of them will give identities as a result.

For x equal to one, we get 1 4 + 1 3 + 2 1 2 - 1 - 3 \u003d 0, which means that one will be the root of this equation.

Now let's divide the polynomial x 4 + x 3 + 2 x 2 - x - 3 by (x - 1) into a column:

So x 4 + x 3 + 2 x 2 - x - 3 = x - 1 x 3 + 2 x 2 + 4 x + 3.

1 3 + 2 1 2 + 4 1 + 3 = 10 ≠ 0 (- 1) 3 + 2 (- 1) 2 + 4 - 1 + 3 = 0

We got an identity, which means we found another root of the equation, equal to - 1.

We divide the polynomial x 3 + 2 x 2 + 4 x + 3 by (x + 1) in a column:

We get that

x 4 + x 3 + 2 x 2 - x - 3 = (x - 1) (x 3 + 2 x 2 + 4 x + 3) = = (x - 1) (x + 1) (x 2 + x + 3)

We substitute the next divisor into the equation x 2 + x + 3 = 0, starting from - 1:

1 2 + (- 1) + 3 = 3 ≠ 0 3 2 + 3 + 3 = 15 ≠ 0 (- 3) 2 + (- 3) + 3 = 9 ≠ 0

The resulting equalities will be incorrect, which means that the equation no longer has integer roots.

The remaining roots will be the roots of the expression x 2 + x + 3 .

D \u003d 1 2 - 4 1 3 \u003d - 11< 0

It follows from this that this square trinomial has no real roots, but has complex conjugate ones: x = - 1 2 ± i 11 2 .

Let us clarify that instead of dividing into a column, Horner's scheme can be used. This is done like this: after we have determined the first root of the equation, we fill in the table.

In the table of coefficients, we can immediately see the coefficients of the quotient from the division of polynomials, which means x 4 + x 3 + 2 x 2 - x - 3 = x - 1 x 3 + 2 x 2 + 4 x + 3.

After finding the next root, equal to - 1 , we get the following:

Answer: x \u003d - 1, x \u003d 1, x \u003d - 1 2 ± i 11 2.

Example 2

Condition: solve the equation x 4 - x 3 - 5 x 2 + 12 = 0.

Decision

The free member has divisors 1 , - 1 , 2 , - 2 , 3 , - 3 , 4 , - 4 , 6 , - 6 , 12 , - 12 .

Let's check them in order:

1 4 - 1 3 - 5 1 2 + 12 = 7 ≠ 0 (- 1) 4 - (- 1) 3 - 5 (- 1) 2 + 12 = 9 ≠ 0 2 4 2 3 - 5 2 2 + 12 = 0

So x = 2 will be the root of the equation. Divide x 4 - x 3 - 5 x 2 + 12 by x - 2 using Horner's scheme:

As a result, we get x - 2 (x 3 + x 2 - 3 x - 6) = 0 .

2 3 + 2 2 - 3 2 - 6 = 0

So 2 will again be a root. Divide x 3 + x 2 - 3 x - 6 = 0 by x - 2:

As a result, we get (x - 2) 2 (x 2 + 3 x + 3) = 0 .

Checking the remaining divisors does not make sense, since the equality x 2 + 3 x + 3 = 0 is faster and more convenient to solve using the discriminant.

Let's solve the quadratic equation:

x 2 + 3 x + 3 = 0 D = 3 2 - 4 1 3 = - 3< 0

We get a complex conjugate pair of roots: x = - 3 2 ± i 3 2 .

Answer: x = - 3 2 ± i 3 2 .

Example 3

Condition: find the real roots for the equation x 4 + 1 2 x 3 - 5 2 x - 3 = 0.

Decision

x 4 + 1 2 x 3 - 5 2 x - 3 = 0 2 x 4 + x 3 - 5 x - 6 = 0

We perform the multiplication 2 3 of both parts of the equation:

2 x 4 + x 3 - 5 x - 6 = 0 2 4 x 4 + 2 3 x 3 - 20 2 x - 48 = 0

We replace the variables y = 2 x:

2 4 x 4 + 2 3 x 3 - 20 2 x - 48 = 0 y 4 + y 3 - 20 y - 48 = 0

As a result, we got a standard equation of the 4th degree, which can be solved according to the standard scheme. Let's check the divisors, divide and in the end we get that it has 2 real roots y \u003d - 2, y \u003d 3 and two complex ones. We will not present the entire solution here. By virtue of the replacement, the real roots of this equation will be x = y 2 = - 2 2 = - 1 and x = y 2 = 3 2 .

Answer: x 1 \u003d - 1, x 2 \u003d 3 2

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