0~π/2 |cosx|=cosxπ/2~π |cosx|=-cosx -cos(x+π/2)=sinx π~3π/2 |cosx|=-cosx -cos(x+π)=cosx3π/2~2π |cosx|=cosx cos(x+3π/2)=sinx ∫[0, π/2]f(sinx)dx=∫[0, π/2]f(cosx)dx
