∫(sinx)^3/cosx dx
=∫(1-(cosx)^2) sinx dx/cosx
cosx=t , -sinx dx=dt
∫(1-t^2)(-dt)/t
=∫(t-1/t)dt
=t^2/2 -log│t│＋Ｃ
=(cosx)^2/2 -log│cosx│＋Ｃ