Z对x偏导=f'(u+v)*(u对x偏导+v对x偏导)+g'(v)*(v对x偏导)
u^2=(x+y)/2.v^2=(x-y)/2.所以u对x偏导等于1/2√[2(x+y)] v对x偏导等于1/2√[2(x-y)]
然后代入原来的式中即可.