(1+tg^2a+1/sin^2a)*sin^2a*cos^2a=1
(1/cos²α+1/sin²α)*sin²αcos²α=1
(sin²αcos²α)/cos²α+(sin²αcos²α)/sin²α=1
sin²α+cos²α=1
(1+tg^2a+1/sin^2a)*sin^2a*cos^2a=1
(1/cos²α+1/sin²α)*sin²αcos²α=1
(sin²αcos²α)/cos²α+(sin²αcos²α)/sin²α=1
sin²α+cos²α=1