(x-y)²=225 x²-2xy+y²=225
(x+y)²=17 x²+2xy+y²=17
下式减上式,得 4xy = (17-225) =-208
因此 xy=-52对不起，打错了，(x-y)²=25(x-y)²=25 x²-2xy+y²=25(x+y)²=17x²+2xy+y²=17下式减上式，得 4xy = (17-225) =-8因此 xy=-2