1/1×2+1/2×3+1/3×4+......+1/n(n+1)
=1-1/2+1/2-1/3+...+1/n-1/(n+1)
=1-1/(n+1)
=n/(n+1)
因为 1/n(n+1)= 1/n - 1/n+1
所以1/1×2+1/2×3+1/3×4+......+1/n(n+1)
=(1 - 1/2) +(1/2 - 1/3)+...........+ 1/n - 1/n+1
=1- 1/n+1
=n/n+1
x=15/36乘以8/15x=2/9
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