9231 P13 - Jun 2013 - Q1 - 5 marks
6397
Let \(\mathrm{f}(r)=r!(r-1)\). Simplify \(\mathrm{f}(r+1)-\mathrm{f}(r)\) and hence find \(\sum_{r=n+1}^{2 n} r!\left(r^{2}+1\right)\).
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