1n(n+1)+1(n+1)(n+2)+1(n+2)(n+3)+1(n+3)(n+4)
=1/n-1/(n+1)+1/(n+1)-1/(n+2)-1/(n+2)-1/(n+3)+1/(n+3)-1/(n+4)
=1/n-1/(n+4)
=4/n(n+4)
1\n(n+1)+1\(n+1)(n+2)+1\(n+2)(n+3)+1\(n+3)(n+4)
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