Question

(tg²a-sin²a)*ctg²a=sin²a
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Answer 1

$(tg^2 alpha -sin^2 alpha)*ctg^2 alpha =sin^2 alpha ;$
$tg^2 alpha*ctg^2 alpha -sin^2 alpha*ctg^2 alpha =sin^2 alpha ;$
$1 -sin^2 alpha* frac{cos^2 alpha }{sin^2 alpha} =sin^2 alpha ;$
$1 -cos^2 alpha =sin^2 alpha ;$
$sin^2 alpha =sin^2 alpha ;$