1/(cosx)^3 = (secx)^3
∫(secx)^3dx = ∫secxdtanx
=secx. tanx - ∫secx.(tanx)^2dx
=secx. tanx - ∫secx.[(secx)^2-1]dx
2∫(secx)^3dy =secx. tanx + ∫secxdx
=secx. tanx + ln|secx+tanx|
∫(secx)^3dy = (1/2)[secx. tanx + ln|secx+tanx| ] + C
1/cosx^3原函数=(1/2)[secx. tanx + ln|secx+tanx| ] + C