Question

((sin a)/(tg a))^2+((cos a)/(ctg a))^2-2sin^2a

Answer 1

$(frac{sina}{tga})^{2}+(frac{cosa}{ctga})^{2}-2sin^{2}a=(frac{sina}{frac{sina}{cosa}})^{2}+(frac{cosa}{frac{cosa}{sina}})^{2}-2sin^{2}a =$

$=cos^{2}a+sin^{2}a-2sin^{2}a=$$cos^{2}a-sin^{2}a=cos2a$