Question 29·Medium·Circles
In a circle with radius centimeters, the length of arc is centimeters. What is the measure of the central angle that intercepts arc , in degrees?
For arc length questions, first decide whether to use the formula (with in radians) or the proportion . Plug in the radius and arc length, solve carefully for the angle (in radians or as a fraction of the full circle), and if you use radians, always multiply by to convert to degrees. Keep track of fractions and simplify step by step to avoid small arithmetic mistakes that lead to close but incorrect answer choices.
Hints
Connect arc length to angle size
Ask yourself: how are arc length, the circle’s radius, and the central angle related? Think of a formula that uses all three.
Use a formula or a proportion
You can either use with in radians, or use a proportion with the full circumference: .
Set up the equation with the given numbers
The radius is 10, so the circumference is . Plug and into your chosen formula or proportion, then solve step by step for the angle. If you used radians, remember to convert to degrees at the end.
Desmos Guide
Compute the central angle directly
In Desmos, type the expression (8*pi/10)*(180/pi). The value that Desmos returns is the measure of the central angle in degrees.
Step-by-step Explanation
Relate arc length, radius, and central angle
For a circle, the relationship between arc length , radius , and central angle (in radians) is
Here, centimeters and centimeters. We first solve for in radians.
Solve for the central angle in radians
Use and plug in the given values:
Divide both sides by 10:
So the central angle is radians. Now convert this to degrees.
Convert the angle from radians to degrees and choose the answer
To convert radians to degrees, multiply by :
The cancels, and you get
So the measure of the central angle is degrees, which corresponds to choice B.