Triangular Number Pairs with Triangular Sum and Difference/Examples/T39, T44/Mistake

Source Work

 * The Dictionary
 * $780$
 * $780$

Mistake

 * $780$ and $990$ are the $2$nd smallest pair of triangular numbers whose sum and difference ($1770$ and $210$) are also triangular.

It is in fact the $4$th such pair, not the $2$nd, after $\left({15, 21}\right)$, $\left({105, 171}\right)$, $\left({378, 703}\right)$.

Also see

 * Triangular Number Pairs with Triangular Sum and Difference/Mistake, where the source of the error is explained.