Let $\triangle{ABC}$ be a right triangle where $\angle{C} = 90^\circ$. If point $D$ is on side $BC$ or its extension, show that $$AB^2 = DB^2 + DA^2 \pm 2 \cdot DB \cdot DC$$ If $D$ is on $BC$, then the $3^{rd}$ term above takes a positive coefficient. Otherwise, if $D$ is on its extension, it takes a negative coefficient.