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Question #df465

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Feb 11, 2017

$LHS=cos(3a+a)$

$=cos(2xx2a)$

$=2cos^2 2a-1$

$=2(2cos^2a-1)^2-1$

$=2(4cos^4a-4cos^2a+1)-1$

$=8cos^4a-8cos^2a+2-1$

$=1-8cos^2a+8cos^4a=RHS$

Proved

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