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# Gimmicky Bidding Websites (Series/Sequence) The way this site works is that if you bid, you have to pay that amount, even if you lose.  Bids are incremented by 1 cent.   Let’s say an item sells for 10 cents.  The guy who bid 1 cent still has to pay that, the guy who bid 2 cents still has to pay that, and so forth.   So, what does the auction site actually earn for selling that item for 10 cents?

Notice that 1+2+3+4+5+6+7+8+9+10  can be added up by grouping numbers from the opposite ends:  $$(1+10) + (2+9) + (3+8) + (4+7) + (5+6)$$   This is just $$11+11+11+11+11$$  or 11*5 = 55.  Note that when x = 10, and we ended up multiplying $$11*5$$ for the series sum.

So, the general formula is:

1 + 2 + 3 + … + n = $$\displaystyle\sum\limits_{x=0}^n x = (n+1)\frac{n}{2} = \frac{n^2+n}{2} = \frac{n(n+1)}{2}$$

Pop Quiz!  If the sunglasses in the photo end up selling for \$6.96, how much does the website make?  $$\frac{696*697}{2}$$ = $$\frac{485112}{2}$$ = $$242556$$ = $$\2,425.56$$   !!