1/2 (1+tg^2x)/tgx, dx = 1/2 {1/tgx+tgx} \, dx = =1/2(ln|sinx|-ln|cosx|)=
=1/2ln|tgx|+C
F(П/4)=1/2ln1+C=5 C=5
F(x)=1/2ln|tgx|+5
