Question 23·Medium·Area and Volume
A circle has radius . A sector of the circle has a central angle of . What is the area, in square centimeters, of the sector?
For sector area questions, immediately write the formula or think of it as “fraction of circle’s angle times area of full circle.” Plug in the given radius and central angle, simplify the fraction first, then multiply by and . Always double-check that you used the radius (not the diameter) and that the angle is treated as a fraction of 360°, which keeps the arithmetic quick and reduces mistakes.
Hints
Relate the sector to the whole circle
Think about how the area of a sector compares to the area of the whole circle. How does the central angle of the sector (in degrees) help you find what fraction of the circle it represents?
Use the sector area formula
Recall the formula for the area of a sector of a circle with radius and central angle (in degrees): it involves the fraction multiplied by the area of the full circle .
Plug in and simplify carefully
Substitute and into the sector area formula. Simplify and reduce the fraction before multiplying to keep the arithmetic easy.
Desmos Guide
Use Desmos to compute the sector area
In Desmos, type the expression (150/360)*pi*12^2 and press Enter. Compare the simplified symbolic result shown by Desmos to the answer choices; the matching expression is the correct choice.
Step-by-step Explanation
Write the formula for the area of a sector
For a circle with radius and a sector with central angle degrees, the area of the sector is
This formula says: take the fraction of the full circle () and multiply by the area of the whole circle ().
Substitute the given radius and angle
Here, the radius is centimeters and the central angle is .
Substitute these into the formula:
Calculate :
So the expression becomes
Simplify the fraction and multiply
First reduce the fraction :
Now multiply this by :
Compute , so
So the area of the sector is square centimeters, which corresponds to choice C.