Posted by **anonymous** on Sunday, May 2, 2010 at 5:20pm.

I need help solving this problem algebraically, but I need to learn step by step!!

Thanks so much

cos 2x=cos x

How do I do it?

- MATH ANALYSIS -
**Reiny**, Sunday, May 2, 2010 at 5:42pm
you should know that

cos 2x = 2cos^2 x - 1

so we have

2cos^2 x - 1 = cosx

2cos^2 x - cosx - 1 = 0

(2cosx + 1)(cosx -1) = 0

cosx = -1/2 or cosx = 1

x = 120° or 240° or x = 0 or 360

in radians

x = 0, 2π/3, 4π/3, 2π

- MATH ANALYSIS -
**anonymous**, Sunday, May 2, 2010 at 5:54pm
Thanks!

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