z是x 的函数,于是u 是x 和y 的函数.
z=sin(x+z) => z ' = cos(x+z) ( 1+ z ' ) => dz/dx = cos(x+z) / [ 1- cos(x+z) ]
u= F(x,y,z) = x(z+y),
δu/δx = δF/δx + δF/δz * dz/dx = z+y + x * cos(x+z) / [ 1- cos(x+z) ]
δ²u/δxδy = δ (δu/δx) /δy = 1