已知x+y=5,xy=3,求x^2+y^2的值和x-y的值已知x-y=5,x^2+y^2=9求xy的值和x+y的值已知x^2+y^2=4,xy=-2,x小于y,求x^2-y^2的值

Question

已知x+y=5,xy=3,求x^2+y^2的值和x-y的值
已知x-y=5,x^2+y^2=9 求xy的值和x+y的值
已知x^2+y^2=4,xy=-2,x小于y,求x^2-y^2的值

Answer 1

x^2+y^2
=(x+y)²-2xy
=5×5-2×3
=19
（x-y）^2
=x²+y²- 2xy
=19-2×3
=13
所以：
x-y=±√13
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