Question

which formula finds area of a circle?
$a=2\pi r$
$a=\pi d$
$a=\pi r$
$a=\pi r^{2}$

Explanation:

• $A=2\pi r$ is the circumference formula of a circle.
• $A=\pi d$ is also a circumference formula (since $d=2r$).
• $A=\pi r$ is an incorrect, non-standard formula for circles.
• $A=\pi r^2$ is the standard formula for the area of a circle, where $r$ is the radius of the circle.

Answer:

Red box: $A=\pi r^2$