Verified answer

1+tg^2a=1/cos^2a

(4(cosa/sina)/(sina/cosa+cosa/sina))*1/cos^2a=

=4(cosa/sina)(1/cos^2a)(sinacosa)=4

1+tg^2alfa=1/cos^2 alfa

tgalfa+ctgalfa=sinalfa/cosalfa+cosalfa/sinalfa=(sin^2 alfa+cos^2 alfa)/sinalfa*cosalfa

(sin^2 alfa+cos^2 alfa)/(sinalfa*cosalfa*cos^2 alfa)=1/sinalfa*cos^3 alfa

4cosalfa/sinalfa*sinalfa*cos^3 alfa=4/sin^2 alfa*cos^2 alfa