## Geometry: Common Core (15th Edition)

Find the first few sums and look for a pattern 1 term, 2=2 2 terms 6=2+4 3 terms 12=9+3 4 terms 20=16+4 Each sum is the product of the number of the terms and the number of terms plus 1. The value of the $n^{th}$ term is $n(n+1)$, so the 100th term will be $100\times101=10100$.