设f(x+y，x-y)=x2-y2，则f(x，y)=()

设f(x+y，x-y)=x²-y²，则f(x，y)=()
A、x²-y²
B、x²+y²
C、(x-y)²
D、xy
【正确答案】：D
【题目解析】：多元函数．f(x+y，z-y)=x²-y²=(x+y)(x-y)，令u=x+y，υ=x-y，则f(x+y，x-y)=f(u，υ)=u，即f(x，y)=xy．