已知(x+y)²=7,(x-y)²=5,则x²+y²= xy=
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已知(x+y)²=7,(x-y)²=5,则x²+y²= xy=
![已知(x+y)²=7,(x-y)²=5,则x²+y²= xy=](/uploads/image/z/17027498-2-8.jpg?t=%E5%B7%B2%E7%9F%A5%EF%BC%88x%2By%29%26%23178%3B%3D7%2C%28x-y%29%26%23178%3B%3D5%2C%E5%88%99x%26%23178%3B%2By%26%23178%3B%3D+xy%3D)
(x+y)²=7
x²+y²+2xy=7 (1)
(x-y)²=5
x²+y²-2xy=5 (2)
(1)+(2)式得:
2x²+2y²=12
x²+y²=6
(1)-(2)式得:
4xy=2
xy=1/2
x²+y²+2xy=7 (1)
(x-y)²=5
x²+y²-2xy=5 (2)
(1)+(2)式得:
2x²+2y²=12
x²+y²=6
(1)-(2)式得:
4xy=2
xy=1/2
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