let
y = secx
dy = secxtanx dx
∫(tanx)^3(secx)^3 dx
= ∫(tanx)^2(secx)^2 dsecx
=∫ [(secx)^2 - 1] (secx)^2 dsecx
= (secx)^5/5 - (secx)^3/3 + C #