The area of a circle is 78.5, what is its radius?

Answer 1

HI,

Nearly all math problems test three things:

(1) Do you know the definitions ?

(2) Do you know the formulas ?

(3) Do you know the processes?

This question checks

(1) What is definition of area, circle, and radius? Also, what units does each have?

(2) What is the formula for the area of a circle? [note: not formula for circumference of circle]

(3) How do you compute radius, given area? (or given circumference; or given diameter; ,...)?

= = = = =

(1) area of a circle is measured in square inches, square miles, square centimeters, ...

(2) A = Πr^r

(3) So, r = Sqrt(A/η)

r = Sqrt(78.5/3.14)

r = Sqrt(25)

radius = 5

Notes: Units were not given. (-5) is not a possible measurement, although (-5)² = 25.

Also note that 3.14 is only an approximation of Π.

Answer 2

Hello, Myah,

The area of a circle is related to the circle's radius by the formula:

Area = πr²

There are no units given for the 78.5, but I'll assume cm².

Therefore: 78.5cm² = πr²

r² = 78.5cm²/π

r² = 25.0 cm²

r = ±5 cm

We'll chose r = 5 cm as the only valid answer.

Bob

Answer 3

A = πr²

78.5 = πr²