Question

1-ctg2a*tga/tga ctga

Answer 1

1-ctg2a*tga/tgactga=1-ctg2a*tga=1-1/tg2a*tga=1-(1-tg²a)/2tga*tga=1-(1-tg²a)/2=
=1-1/2+1/2*tg²a=1/2+1/2*tg²a=1/2(1+tg²a)=1/2*1/cos²a=1/2cos²a