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# Geometric Sequence/Arithmetic Series help watch

1. A child wishes to build up a triangular pile of toy bricks so as to have 1 brick in the top row, 2 in the second, 3 in the third and so on. If he has 100 bricks, how many rows can he complete and how many bricks will he have left over?

I worked out the answer is 13, but my method was using the triangle numbers e.g. 1,3,6,10,15......

Whats the more efficient way?

Thanks
2. Use the formula for the sum of the first n natural numbers: s = n(n+1)/2
set s as 100, find n and round down our answer to get number of rows
then use the rounded down value to find the actual sum, call this number x

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Updated: December 9, 2014
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