(B.)y. (C.)z. (D.)yf(x²-y²).

6.设方程x+z=yf(x²-z²)确定了z=f(x,y)，其中f可微，则$z\frac{\partial z}{\partial x}+y\frac{\partial z}{\partial y}=$（） (
A.)x. (
B.)y. (
C.)z. (
D.)yf(x²-y²).

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