Posted by **bee** on Monday, December 3, 2012 at 10:02pm.

when solving for all solutions how do I work out this problem: cos4(alpha)=cos2(alpha)

and when verifying each identity how do I complete this problem: sin (alpha/2)=sec alpha -1/2sin (alpha/2)sec alpha

- precal -
**Steve**, Monday, December 3, 2012 at 11:53pm
cos4x = cos2x

2cos^2(2x)-1 = cos2x

2cos^2(2x)-cos(2x)-1 = 1

(2cos2x+1)(cos2x-1) = 0

so, cos2x = -1/2 or 1

cos 2pi/3 = cos 4pi/3 = -1/2

cos 0 = 1

as for the identity, I get the feeling there's a typo, because secx appears twice on the right.

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