令 x= √2 sint， 则 I = ∫ <0, √2>√(2-x^2)dx = ∫ <0, π/2> 2(cosx)^2)dx= ∫ <0, π/2> (1-cos2x))dx = [x-(1/2)sin2x] <0, π/2> = π/2
