ACT · Plane Geometry · Polygons, perimeter, and area
Question act-q-00474
Hard Multiple choice No calculator
The question
A circle has radius 9. What is its area, in terms of π?
- 18π
- 9π
- 81π
- 162π
Show answer & worked solution
Answer: 81π
Worked solution. Area of a circle is πr². Here r = 9, so area = π × 9² = 81π.
Why each wrong choice is wrong. Confusing area (πr²) with circumference (2πr) is the dominant error here. The 2πr distractor catches it perfectly.
Test-day tactic. Area scales as the square of the radius; circumference scales linearly. When in doubt, sketch a unit circle and check.
About this question type
Items test areas and perimeters of quadrilaterals, properties of parallelograms and trapezoids, and the interior angle sum of a polygon ((n − 2) × 180°).
You will see a question shaped like this one on roughly every other official ACT form, typically at the back end of the section, where the score gradient is steepest. Treat any miss in this subtopic as a signal to drill the subtopic page before you do another full practice test.