I can prove the first part from formula area of rectangle that it is $(n)(n+1)$ if it triangle it is$\frac{(n)(n+1)}{2}$ But I can't prove the backward part that is true Anyone have a hint ?
2026-05-05 23:23:24.1778023404
A number is triangular if and only if it is of the form $\frac{(n)(n+1)}{2}$
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