What is the measure of each interior angle of a **regular octagon**?

The sum of the interior angles of an $n$-sided polygon is $(n - 2) \cdot 180^{\circ}$. For $n = 8$: sum $= (8 - 2) \cdot 180^{\circ} = 6 \cdot 180^{\circ} = 1{,}080^{\circ}$. A **regular** octagon has all eight interior angles equal, so each is $\dfrac{1{,}080^{\circ}}{8} = 135^{\circ}$. - Trap A ($108^{\circ}$) is the interior angle of a regular pentagon ($n=5$). - Trap B ($120^{\circ}$) is the regular hexagon. - Trap D ($144^{\circ}$) is the regular decagon.