What is the sum of the measures of the interior angles of a **decagon** (a $10$-sided polygon)?

The sum of the interior angles of an $n$-gon is $(n - 2) \cdot 180^{\circ}$. For $n = 10$: $(10 - 2)(180^{\circ}) = 8 \cdot 180^{\circ} = 1{,}440^{\circ}$. - Trap A ($720 = 4 \cdot 180$) is the hexagon ($n=6$). - Trap B ($1080$) is the octagon ($n=8$). - Trap D ($1800$) uses $n$ instead of $n-2$. Quick check: a triangle has $180^{\circ}$, a quadrilateral $360^{\circ}$. Each additional side adds $180^{\circ}$.