Here,
I=intsec^3xdx.....to(A)
I=intsecx(sec^2x)dx
Using Integration by parts:
I=secx color(blue)(intsec^2xdx)-int(secxtanxcolor(blue)( intsec^2xdx))dx
I=secx*color(blue)(tanx)-intsecxtanx*color(blue)(tanx)dx
I=secxtanx-intsecxtan^2xdx
I=secxtanx-intsecx(sec^2x-1)dx
I=secxtanx-intsec^3xdx+intsecxdx
I=secxtanx-I+color(red)(intsecxdx)....to[use,eqn.(A)]
I+I=secxtanx+color(red)(ln|secx+tanx|)+c
2I=secxtanx+ln|secx+tanx|+c
I=1/2[secxtanx+ln|secx+tanx|]+c/2
I=1/2[secxtanx+ln|secx+tanx|]+C ,where, C=c/2