1X2/1+2X3/1+3X4/1+……+9X10/1=_____

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1X2/1+2X3/1+3X4/1+……+9X10/1=_____
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1X2/1+2X3/1+3X4/1+……+9X10/1=_____
1X2/1+2X3/1+3X4/1+……+9X10/1=_____

1X2/1+2X3/1+3X4/1+……+9X10/1=_____
是1/(1*2)+1/(2*3)+1/(3*4)+.+1/(9*10)吧
1/(1*2)+1/(2*3)+1/(3*4)+.+1/(9*10)
=(1/1-1/2)+(1/2-1/3)+……+(1/9-1/10)
=1-1/2+1/2-1/3+……+1/9-1/10
=1-1/10
=9/10