Question

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

Answer 1

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