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# Summing Series FP1 OCR Watch

1. If Sn=n + 2(n-1) + 3(n-2)+...+(n-1)x2 +nx1, where n is a positive integer,
prove that Sn+1 - Sn=1/2(n+1)(n+2)
prove that Sn= 1/6(n)(n+1)(n+2)

I cant understand how to do this?!
2. (Original post by RNVS_1299)
If Sn=n + 2(n-1) + 3(n-2)+...+(n-1)x2 +nx1, where n is a positive integer,
prove that Sn+1 - Sn=1/2(n+1)(n+2)
prove that Sn= 1/6(n)(n+1)(n+2)

I cant understand how to do this?!
For the first part:

Write out the terms for Sn+1 across a page, and underneath write the terms for Sn, in corresponding positions. Then subtract.

NOTE: There are more terms in Sn+1 than in Sn, so take great care with the end parts of the series.

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