identities trig?

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find all solutions to the equation in the interval [0,2pie]

cos2x=cosx

• identities trig? -
• identities trig? -

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

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