X+Y=40,(X*Y)/(X+Y)=7.5
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X+Y=40,(X*Y)/(X+Y)=7.5X+Y=40,(X*Y)/(X+Y)=7.5X+Y=40,(X*Y)/(X+Y)=7.5（x*y）*40=7.5x*y=300因为y=40-x所以x*y=3
X+Y=40,(X*Y)/(X+Y)=7.5
X+Y=40,(X*Y)/(X+Y)=7.5
X+Y=40,(X*Y)/(X+Y)=7.5
（x*y）*40=7.5
x*y=300
因为y=40-x
所以x*y=300
x*（40-x）=300
即x*x-40*x+300=0
（x-30）*（x-10）=0
得x=30或x=10
又因为x+y=40
所以得：x=30时y=10
x=10时y=30
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X+Y=40,(X*Y)/(X+Y)=7.5
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