Posted by emily on Tuesday, April 26, 2011 at 9:47pm.
find all solutions to the equation in the interval [0,2pie]
cos2x=cosx
 identities trig?  MathMate, Tuesday, April 26, 2011 at 9:51pm

identities trig?  Reiny, Tuesday, April 26, 2011 at 9:51pm
cos 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