Question

(sin4-cos4)/(1-cosa)(1+cosa) + 2 ctg^2a= 1/sin^2a

Answer 1

$\dfrac{sin^4a-cos^4a}{(1-cosa)(1+cosa)}+2ctg^2a= \\ \\ = \dfrac{(sin^2a-cos^2a)(sin^2a+cos^2a)}{1-cos^2a}+2ctg^2a = \\ \\ = \dfrac{sin^2a-cos^2a}{sin^2a}+ \dfrac{2cos^2a}{sin^2a} = \\ \\ = \dfrac{sin^2a+cos^2a}{sin^2a}= \\ \\ = \dfrac{1}{sin^2a}$