Solution
Triangular numbers will be given $$n=\frac{n\left(n+1\right)}{2}$$
1st triangular numbers = n=1 then $$=\frac{1\times\ 2}{2}=1$$
2nd triangular numbers = n=2 then $$=\frac{2\times\ 3}{2}=3$$
Similarly we will find
First five triangular numbers 1, 3, 6, 10, 15
Average = $$\frac{1+3+6+10+15}{5}=7$$.
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