(x＋y)²＝1 (x－y)²=49 求x²＋y²与xy的值

(x＋y)²＝1 (x－y)²=49 求x²＋y²与xy的值
(x＋y)²＝1 (x－y)²=49 求x²＋y²与xy的值

(x＋y)²＝1 (x－y)²=49 求x²＋y²与xy的值
(x+y)²=1 x²+2xy+y²=1 ①
(x-y)²=49 x²-2xy+y² =49 ②
①+② 2(x²+y²) =50
x²+y² =25
带入① 2xy=-24
xy=-12