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## 'Triangle Numbers' printed from http://nrich.maths.org/

The first triangle number is $1$

The second triangle number is $1 + 2$

The third triangle number is $1 + 2 + 3$

The fourth triangle number is $1 + 2 + 3 + 4 \ldots$

The $n^{\text{th}}$ triangle number is $\frac{n(n+1)}{2}$