2.设 (xy,x+y)=(x)^2+(y)^2+xy (其中, =xy =x+y), 则 dfrac (partial f)(partial u)+dfrac (partial f)(partial v)= __ 。。-|||-A. 2(x+y) B. (x+y)-|||-C. 2(x+y)-1 D. 1-2(x+y)-|||-山-|||-正确