then by use of ur method by subing in the trig. values then :-
the intergal of tan^3x.sec^2x dx would be
intergal of tan^3 x.tan^2 x+1 dx
Intergal of tan^5 x +tan^3 x dx
=tan^6 x/6 + tan^4 x/4 +C
which its the answer only tan^4 x/4 is the answer
method doesnt work.