Question

2cos^2a-(tga*cosa)^2-(ctga*sina)^2

Answer 1

$2cos^{2}a-(tga*cosa)^{2}-(ctga*sina)^{2}=2cos^{2}a-(frac{sina}{cosa}*cosa)^{2}-</var><var>(frac{cosa}{sina}*sina)^{2}=2cos^{2}a-sin^{2}a-cos^{2}a=cos^{2}a-sin^{2}a=cos2a$