(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

cos4x-7cos2x=8 solve in interval [0,2[tex]\pi[/tex])

2. Relevant equations

reciprocal identities, pythagorean identities, sum/difference formulas, double angle formulas, power reducing formula, half-angle formula, product to sum formula, sum to product formula.

3. The attempt at a solution

So to solve this problem I am pretty sure I will have to use the double angle formula. However, I am a little confused as to how the formula changes when you have more than 2x. the only additional example my textbook gives it Cos6x=cos[tex]^{}2[/tex]3x-sin[tex]^{}2[/tex]3x. So from that I can assume that cos4x= cos[tex]^{}2[/tex]2x-sin[tex]^{}2[/tex]2x. But how would cos2x=2cos[tex]^{}2[/tex]x-1 change when its cos4x? I got my problem down to:

cos4x-14cos[tex]^{}2[/tex]x-15

I'm hoping that if I can expand on cos4x then I can maybe factor...or am I going about this all wrong.

Please help, I hate to be a bother but I'm taking this Pre-cal class independently over the summer and have already got an incomplete grade. My teacher said I have until the 20th to finish all the test and labs, so I'm trying to tie up all my loose ends so I can be done and move onto calculus.

Thanks

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# Homework Help: Trig problem cos4x-7cos2x=8

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