Friday, May 10, 2013

Prove the following Identity? CosA cos(-A)- sinAsin(-A)=1


in the equation 

%0D%0Acos%28A%29+cos%28-A%29-+sin%28A%29sin%28-A%29=1%0D%0A 

we notice the evenness and oddness of cosine and sine:
%0D%0Acos%28-theta%29=cos%28theta%29%0D%0A 
and
%0D%0Asin%28-theta%29=-sin%28theta%29%0D%0A 

therefore...
%0D%0Acos%28A%29+cos%28-A%29-+sin%28A%29sin%28-A%29=cos%5E2%28A%29%2Bsin%5E2%28A%29=1%0D%0A

:)
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