Trigonometric Equations - Why Do I Always Get cos^3 or sin^3?

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SUMMARY

This discussion focuses on solving cubic trigonometric equations involving cos^3(x) and sin^3(x). The participants suggest transforming the equations by substituting sin²(x) with 1 - cos²(x) and vice versa, leading to cubic equations in cos(x) and sin(x). To solve these cubic equations, they recommend applying the Rational Root Theorem to identify potential rational roots, which can simplify the problem to a quadratic equation. This method is essential for students struggling with cubic trigonometric equations, especially when such topics are not covered in their textbooks.

PREREQUISITES
  • Understanding of trigonometric identities, specifically sin²(x) and cos²(x).
  • Familiarity with cubic equations and polynomial functions.
  • Knowledge of the Rational Root Theorem for polynomial equations.
  • Basic algebraic manipulation skills for simplifying equations.
NEXT STEPS
  • Study the application of the Rational Root Theorem in polynomial equations.
  • Learn methods for solving cubic equations, including synthetic division.
  • Explore trigonometric identities and their applications in solving equations.
  • Research techniques for converting cubic trigonometric equations into quadratic forms.
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Students studying trigonometry, mathematics educators, and anyone seeking to enhance their problem-solving skills in cubic equations and trigonometric functions.

okh
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Homework Statement


I'm trying to solve two similar equations, but I can't go on.
This is the first one
http://img818.imageshack.us/img818/298/imageevsu.jpg

This is the second one:
http://img7.imageshack.us/img7/7812/imageowud.jpg

Homework Equations





The Attempt at a Solution


For the first one:
http://img855.imageshack.us/img855/4633/imagekhd.jpg

And for the second equation:
http://img12.imageshack.us/img12/2990/imageiag.jpg

And I'm stuck with these cos^3x and sin^3x. I think I should replace something...
 
Last edited by a moderator:
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okh said:

Homework Statement


I'm trying to solve two similar equations, but I can't go on.
This is the first one
http://img818.imageshack.us/img818/298/imageevsu.jpg

This is the second one:
http://img7.imageshack.us/img7/7812/imageowud.jpg

Homework Equations



The Attempt at a Solution


For the first one:
http://img855.imageshack.us/img855/4633/imagekhd.jpg

And for the second equation:
http://img12.imageshack.us/img12/2990/imageiag.jpg

And I'm stuck with these cos^3x and sin^3x. I think I should replace something...
For the first one, change sin2(x) to 1-cos2(x). You will have a cubic equation in cos(x).

For the second one, change cos2(x) to 1-sin2(x). You will have a cubic equation in sin(x).
 
Last edited by a moderator:
Okay, but how to solve cubic trigonometic equations?
I think I can get a quadratic equation, in some way. We are studying them, indeed, and there is nothing about cubic trigonometric equations in my book.
 
Well, if you don't do something you can't possibly get an answer!

Have you tried just checking some simple values? It helps sometimes to know the "rational root theorem". Any rational root, of the form p/q with integers p and q, of the polynoial a_nx^n+ a_{n-1}x^{n-1}+ a_1x+ a_0= 0 must have p evenly dividing a_0 and q evenly dividing a_n. That will reduce the number of trials. Of course, it is not necessary that a polynomial equation have rational roots but this one does.

Once you have a single root, say x= a, divide the cubic polynomial by x- a to reduce to a quadratic.
 

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