∫(0,π) cos²(x/2) dx= ∫(0,π) (1 + cosx)/2 dx= (1/2)∫(0,π) (1 + cosx) dx= (1/2)(x + sinx)|(0,π)= (1/2)(π + 0)= π/2公式：cos2x = 2cos²x - 1cos²x = (1 + cos2x)/2
