About 100 C.E., Plutarch noted that if a triangular number ($\displaystyle {T}_{{n}}$) is multiplied by 8 and then 1 is added to the result, the resulting value will be a perfect square number. What perfect square results from applying Plutarch's observation to $\displaystyle {T}_{{13}}.$

Answer: The square number is
You should make sure the number you get is a perfect square before entering it.

Plutarch

(Source: http://www.livius.org/aj-al/alexander/alexander_z1b.html