y = x²,x = y²联立的交点为(0,0),(1,1)在x处(0 < x < 1):旋转体为外径为y = √x,内径为y = x²的圆环,截面积为π(√x)² - π(x²)²的圆环．旋转体体积为π(√x)² - π(x²)²在[0,1]上的积分,结果为π(1/2 - 1/5) = 3π/10