(2)由定义可知: Z u(Mo, to)=lim u(r, t) r→>0 → t0 M0(x0,y0, 要求v(M0,t0) 必须求u(r,t) x t rsin e cos yo+rsin 8 sin r cOS 6 (x-x)2+(y-y0)2+(x2-=)(2) 由定义可知： ( , ) ( , ) lim 0 0 0 0 u M t u r t t t r ® ® = ( , ) ( , ) 0 0 u r t u M t 必须求 \ 要求 ï î ï í ì = + = + = + q q j q j cos sin sin sin cos 0 0 0 z z r y y r x x r 2 0 2 0 2 0 r = ( x - x ) + ( y - y ) + (z - z ) ( , , ) 0 0 0 0 M x y z O Z Y r j M (x, y,z)