1/cos^3(x) =can(1+tan^2(x))/cox^2(x) ,do 1+tan^2(x) =1/cos^2(x),tu do dat t=tan(x), dua tich phan ve tich phan can(1+t^2)dt(dat =I)
can(1+t^2)=1/can(1+t^2) +t^2/can(1+t^2) tach thanh tong 2 tich phan do ,cai dau tien dat an phu u=t+can(1+t^2) , ket qua la ln(t+can(t^2+1) +c1, cai thu 2 dung tung phan dat u1=t , dv1=t/can(1+t^2)dt suy ra v1=can(1+t^2) , tiep tuc se dua den ket qua co dang truy hoi tuc la cai ban dau la I = (...) -I , suy ra I=(..)/2 ,trong dau(...) la ln(t+can(1+t^2)) +c1+tcan(t+t^2) .