请在这里概述您的问题已知(x+y)²=21,(x-y)²=15,求(1)x²+y²的值 (2)xy的值急
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请在这里概述您的问题已知(x+y)²=21,(x-y)²=15,求(1)x²+y²的值 (2)xy的值急
请在这里概述您的问题已知(x+y)²=21,(x-y)²=15,求(1)x²+y²的值 (2)xy的值
急
请在这里概述您的问题已知(x+y)²=21,(x-y)²=15,求(1)x²+y²的值 (2)xy的值急
(x+y)²=21
(x-y)²=15
分开:
x²+2xy+y²=21 (1)
x²-2xy+y²=15 (2)
(1)+(2):
2(x²+y²)=36
x²+y²=18
(1)-(2):
4xy=6
xy=1.5
所以:
x²+y²=18
xy=1.5
(x+y)²=21,(x-y)²=15
所以
x² + 2xy + y² =21 --- (1)
x² - 2xy + y² = 15 --- (2)
由(1) + (2)
2(x² + y²) = 36, x² + y² = 18.
由(1) - (2)
4xy = 6, xy = 3/2.