Sunday, July 21, 2013

cos^4a+cos^2a=1 ,prove is it identity or not

not an identity:
To disprove an identity we need only a single exception: 
%0D%0Acos%5E4%28pi%29%2Bcos%5E2%28pi%29=%28-1%29%5E4%2B%28-1%29%5E2=2%0D%0A 
and not 1 

:)
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