Question 16·Easy·Circles
Points A and B lie on a circle with center O. The measure of central angle AOB is 135°, and the circumference of the circle is 32.
What is the length of minor arc AB?
For circle arc-length problems on the SAT, go straight to the proportion . Simplify the angle fraction first (like turning into ) to make multiplication quick and avoid calculator errors. Be careful not to use 180 in place of 360—arc measures in circles always compare to the full .
Hints
Connect arc length and angle
Think about how the length of an arc is related to the circle’s total circumference. How does the measure of the central angle compare to a full circle of ?
Set up a proportion
Try writing a proportion: (arc length)/(circumference) = (central angle)/(360). You know the circumference and the central angle; which part is missing?
Simplify the fraction
Simplify before multiplying by 32. Can you divide both numbers by the same factor to get a simple fraction?
Desmos Guide
Enter the proportion as an expression
In Desmos, type the expression (135/360)*32 to represent the fraction of the circumference for a arc.
Interpret the output
Look at the numerical result that Desmos gives for (135/360)*32; that value is the length of minor arc AB.
Step-by-step Explanation
Relate central angle and arc length
For any circle, the arc length is proportional to the central angle:
Here, we want the length of minor arc , and we know:
- Central angle
- Circumference .
Find the fraction of the circle for 135°
Compute what fraction of the full circle the angle represents:
Now simplify this fraction by dividing numerator and denominator by 45:
So the minor arc AB is of the entire circumference.
Apply the fraction to the circumference
Use the proportion from Step 1 with the fraction from Step 2:
Now multiply by :
- First,
- Then,
So the length of minor arc is 12.