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Posted by **emily** on Tuesday, April 26, 2011 at 9:47pm.

cos2x=cosx

- identities trig? -
**MathMate**, Tuesday, April 26, 2011 at 9:51pm - identities trig? -
**Reiny**, Tuesday, April 26, 2011 at 9:51pmcos 2x = cosx

2cos^2 x - 1 - cosx = 0

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

cosx = -1/2 or cosx = 1

if cosx = -1/2, x is in II or III

x = 2π/3 or 4π/3 ......I knew cos π/3 = +1/2

if cosx = 1, then x = 0 or x = 2π

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