1/1乘5+1/5乘9+1/9乘13……+1/101乘105怎么算?

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1/1乘5+1/5乘9+1/9乘13……+1/101乘105怎么算?
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1/1乘5+1/5乘9+1/9乘13……+1/101乘105怎么算?
1/1乘5+1/5乘9+1/9乘13……+1/101乘105怎么算?

1/1乘5+1/5乘9+1/9乘13……+1/101乘105怎么算?
1/1x5+1/5x9+1/9x13……+1/101x105
=1/4(1-1/5+1/5-1/9+1/9-1/13+……+1/101-1/105)
=1/4(1-1/105)=1/4 x 104/105
=26/105
【1/n(n+m)=1/m x (1/n-1/(n+m))】
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裂项1/[n(n+4)]=[1/n-1/(n+4)]*1/4
所以1/(1*5)+...+1/(101*105)
=1/4*(1-1/5+1/5-1/9+....+1/101+1/105)
=1/4*(1-1/105)
=1/4*104/105
=26/105