1.若x+y=7,xy=12,求x²+y²=?2.若x+y=7,x²+y²=,xy=?3.若a²+b²=12,ab=12,求a+b=?
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![1.若x+y=7,xy=12,求x²+y²=?2.若x+y=7,x²+y²=,xy=?3.若a²+b²=12,ab=12,求a+b=?](/uploads/image/z/3169834-34-4.jpg?t=1.%E8%8B%A5x%2By%3D7%2Cxy%3D12%2C%E6%B1%82x%26%23178%3B%2By%26%23178%3B%3D%3F2.%E8%8B%A5x%2By%3D7%2Cx%26%23178%3B%2By%26%23178%3B%3D%2Cxy%3D%3F3.%E8%8B%A5a%26%23178%3B%2Bb%26%23178%3B%3D12%2Cab%3D12%2C%E6%B1%82a%2Bb%3D%3F)
1.若x+y=7,xy=12,求x²+y²=?2.若x+y=7,x²+y²=,xy=?3.若a²+b²=12,ab=12,求a+b=?
1.若x+y=7,xy=12,求x²+y²=?
2.若x+y=7,x²+y²=,xy=?
3.若a²+b²=12,ab=12,求a+b=?
1.若x+y=7,xy=12,求x²+y²=?2.若x+y=7,x²+y²=,xy=?3.若a²+b²=12,ab=12,求a+b=?
1
x+y=7
两边平方:
x²+y²+2xy=49
x²+y²=49-2*12=25
2
少条件,不能算.
3
a²+b²=12
2ab=2*12
两式相加:
a²+b²+2ab=36
(a+b)²=36
两边开方:
a+b=6或a+b=-6;
x²+y²=(x+y)²-2xy=7*7-2*12=49-24=25 3.(a+b)²=a²+b²+2ab=12+2*12=36 所以a+b=±6
1 x2+y2=(x+y)2-2xy=49-12=37 2 2xy=(x+y)2-(x2+y2) 3 (a+b)2=a2+b2+ab=12+12=24 a+b=正负2根号6