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

Source Work

 * The Dictionary
 * $780$
 * $780$


 * 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 $\tuple {15, 21}$, $\tuple {105, 171}$, $\tuple {378, 703}$.

Also see

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