егор3228 Tgx*ctgx=1 1/-cosx^2=-1/cosx^2 =(-six^2-cosx^2)/cosx^2=-sinx^2/cosx^2 -1 1 - sin^2x +sinx^2/cosx^2+1=2 - sinx^2(cosx^2-1)/cosx^2=2+sinx^4/cosx^2