cos ^{4 }theta - sin ^{4 }theta = cos2 theta
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Is the right hand side cos (2 theta) or cos^2(theta)? Since you used exponents on the left I presume it is the former. In that case, use the fact that cos(2 theta)= 1- 2sin^2(theta) and that cos^2(theta)= 1- sin^2(theta).
Originally Posted by Bonganitedd cos ^{4 }theta - sin ^{4 }theta = cos2 theta I'd treat the LHS as the difference of 2 squares.