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(x+y)(x+y)=1,展開,得xx+2xy+yy=1,代入,得2+2xy=1,得xy=-0.5
因為x+y=5所以(x+y)平方=x平方+2xy+y平方=25所以x平方+y平方=25-3x2=19
xx+yy=(X+Y)(X+Y)-2XY=6*6-2*4=28
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