Question

tg^2 * ctg^2 - cos^2 =

Answer 1

tgx*ctgx=1
tg^2*ctg^2=1
1-cos^2=sin^2

Answer 2

$tg^2 ctg^2 -cos^2=(tg *ctg)^2-cos^2=1^2-cos^2=1-cos^2=sin^2$