x = 0,y = e^0 = 1
x = 1,y = 1/e
绕y轴旋转,用y做自变量较方便:y = e^(-x),x = -lny
0 < y < 1/e时,旋转体为:截面为半径=1,高为1/e的圆柱,体积V1 = π*1²*1/e = π/e
1/e < y < 1处,旋转体截面为以|-lny|为半径的圆,V2 = ∫πln²ydy
= πy(ln²y - 2lny + 2) (1/e ->1)
= π(0 - 0 +2) - π(1 + 2 + 2)/e
= 2π - 5π/e
V = V1 +V2 = π/e + 2π - 5π/e
= 2π - 4π/e查表，哈哈。