曲面∑为上半球面x2+y2+z2=1(z≥0)∑1是第一卦限部分,为什么∫∫zdS=4∫∫xdS

曲面∑为上半球面x2+y2+z2=1(z≥0)∑1是第一卦限部分,为什么∫∫zdS=4∫∫xdS
一次洛必达法则,再使用导数的定义
lim(h→0) [f(x+2h)－2f(x+h)＋f(x)]/h^2
＝lim(h→0) [2f'(x+2h)－2f'(x+h)]/(2h)
＝lim(h→0) [f'(x+2h)－f'(x+h)]/h
＝lim(h→0) ｛2×[f'(x+2h)－f'(x)]/(2h)－[f'(x+h)－f'(x)]/h｝
＝2×lim(h→0)[f'(x+2h)－f'(x)]/(2h)－lim(h→0)[f'(x+h)－f'(x)]/h
＝2×f''(x)－f''(x)
＝f''(x)
直接代入方程
r(x~2+-2+22)ds
= T4 ds
=16
或将方程参数化然后计算
z2+32+A2=4
x+3+2=0
将＝-x-3代入＾2+y～2+2＝4中
==>x2+y~2+xy=2
(x+y/2)2+(V3y/2)~2=2
fa: y/2 v2c0st
f v3y/2 28int
ニ2
(a =v2cost-(v6/3)sint, da =fv2sint-(V6/3)cost
dt
f y=(2v6/3)sint dy= -2v6/3)cost dt
f x=-v2cost -(v6/3)sint, d: v2sint-( v6/3)cos
dt
0st≤2a
ds =VI(da)2 +(dy)42+(dz)] dt v4 dt =2 dt
(x42+3y~2+22)ds
f(0-2)(v2cost-(v6/3)sint]/2 + [(2v6/3)sint]2
1v2cost-(v6/3)sint1 2) *2 dt
(0→2r)4大2dt
=16x