z↑ 10.曲面的面积 zf(x,y) △S.≈△A △A; A;(由引理) osr +fx(x1,y)+∫(x,y)△G 元=士!(x,f(x,)-1有 : S=1+6 2(,y)+/ 2(x,y)drdy (x;,y D10. 曲面的面积 x z y 0 = + + D x y S f (x, y) f (x, y)dxdy i i Ai = cos 1 z =f (x,y) D i Si Ai ni x i i y i i i = 1+ f (x , y ) + f (x , y ) 2 2 . Si (xi , yi ) i Ai (由引理) ( = ( , ), ( , ),−1) i x i i y i i n f x y f x y Pi . .