首先要把f(x,y)求出来，令t=x+y, s=x-y，带人得到f(t,s)=x^2-y^2=(x+y)(x-y)=ts所以f(x,y)= xy然后求偏导得到y+x，答案C
f(x+y, x-y) = (x+y)(x-y)==> f(x,y) = xyf'x + f'y = y + xAnswer: C
