p2(x) A(x)d 给x一增量d,过x+dx做垂直于x轴的平面,从 而得一小曲顶柱体,其体积 △V≈A(x)dx 故 dW=A)dk=国fox,nd 所以 Va-dv-f(x.dvlx 故 f(d= 77 2 1 ( ) ( ) ( ) ( , ) x x A x f x y dy = 给x一增量dx,过x dx + 做垂直于x轴的平面,从 而得一小曲顶柱体,其体积 V A x dx ( ) 故 dV A x dx = = ( ) 2 1 ( ) ( ) [ ( , ) ] x x f x y dy dx 所以 V曲 2 1 ( ) ( ) [ ( , ) ] b b x a a x dV f x y dy dx = = 故 ( , ) D f x y d 2 1 ( ) ( ) [ ( , ) ] b x a x f x y dy dx =