1+tg2 α + 1/sin2 α=1/sin2 α cos2 α
1+tg^2(a)=(cos^2(a)+sin^2(a))/cos^2(a)=1/cos^2(a);
Теперь
1/cos^2(a)+1/sin^2(a)=(cos^2(a)+sin^2(a))/sin^2(a)cos^2(a)=1/(sin^2(a)*cos^2(a)).