Question

Доказать тождество:
sin a +sin 4a/cos 2a - cos 4a= ctg a

Answer 1

ctg^2a-tg^2a=cos^2a/sin^2a-sin^2a/cos^2a=(cos^4a-sin^4a)/cos^2a*sin^2a=cos2a/(sinacosa)^2(cos^2a-sin^2a)/(cos2a)/(sinacosa)^2=sin^2a*cos^2a