1/[n(n+1)(n+2)]=1/2*[1/n-2/(n+1)+1/(n+2)],
例如
∴1/(1*2*3)+1/(2*3*4)+1/(3*4*5)+1/(4*5*6)+1/(5*6*7)+1/(6*7*8)+1/(7*8*9)
=1/2*[1-2/2+1/3+1/2-2/3+1/4+1/3-2/4+1/5+……+1/7-2/8+1/9]
=1/2[1-1/2-1/8+1/9]
=1/2(1/2-1/72)
=35/144.
后面加一项 其实差不多的 自己算算吧
1/24+1/60+1/120+1/210+
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