Fz(x)=‖f(x,y)dxdy y +y=z 化成累次积分,得 F2(x) f(x, y)dx dy 固定z和y,对方括号内的积分作变量代换 =L-y, 得 变量代换 F7(z f(u-y, ydujdy 交换积分次序 f(u-y, ydyldu化成累次积分,得  +  = x y z FZ (z) f (x, y)dxdy    − − − = z y FZ (z) [ f (x, y)dx]dy 固定z和y,对方括号内的积分作变量代换, 令x=u-y,得    − − = − z FZ (z) [ f (u y, y)du]dy −   − = − z [ f (u y, y)dy]du 变量代换 交换积分次序