146.|X-Y|=6, 问XY的最小值可能为?
答案有,0,-16,-9等等
[解]-9 see memo
|X-Y|=6 两边平方---〉x^2 -2xy + y^2 =36, 左边加4xy,再减4xy ---> x^2 +2xy +y^2 -4xy=(x+y)^2 -4xy=36, ---> xy=(x+y)^2/4 -9
最小值当然为-9, 且发生在x=-y的条件下!
X-Y=+-6
1) X-Y=6, XY=(Y+6)*Y=Y^2+6Y, min(XY)=-9
2) X-Y=-6, XY=(Y-6)*Y=Y^2-6Y, min(XY)=-9
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