Solve problems involving the of a circle central angle circumference Solve problems involving Words to Know Write the letter of the definition next to the matching word as you work through the lesson You may use the glossary to help you arc length radian Define measure A a standard unit of measure for angles the measure of a central angle that subtends an arc that is equal in length to the radius of the circle B an angle whose vertex is at the center of a circle and whose sides are radii of that circle BOK C a portion of the circumference of a circle MAGNE AMOR ARRICANE D the distance around a circle