因为：(x+y)²-(x-y)² =x²+2xy+y²-x²+2xy-y² =4xy由条件得 (x+y)²-(x-y)²=25-9=16即 4xy=16所以 xy=16÷4 =4
xy=[(x+y)²-(x-y)²]/4=(25-9)/4=4
