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1、 (x+y)^2=3^2 x^2+2xy+y^2=9 兩邊减去3xy=-30 x^2-xy+y^2=9+30=39 2、 (x-y)^2 =x^2-2xy+y^2 =(x^2-xy+y^2)-xy =39+10 =49 所以|x-y|=7
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