ACT Preparation / Unit 14 ACT 1304 short lesson, about 20 seconds
Circumference and Area of a Circle
Captions available
The idea
A circle's circumference is 2πr, or πd; its area is πr². The diameter is twice the radius, and π is about 3.14.
In practice
A circle with diameter 10 has radius 5, circumference 10π, about 31.4, and area 25π, about 78.5.
Where people slip
Area uses the radius; halve a given diameter before squaring.
Your place on the trail
Every lesson rests on earlier ones and opens later ones. Tap any to jump.
Something wrong in this lesson? Flag it · Sources for ACT Preparation