# High School Math : Radius

## Example Questions

### Example Question #4 : How To Find The Length Of A Radius

The specifications of an official NBA basketball are that it must be 29.5 inches in circumference and weigh 22 ounces.  What is the approximate radius of the basketball?

9.39 inches

4.70 inches

3.06 inches

14.75 inches

5.43 inches

4.70 inches

Explanation:

To Find your answer, we would use the formula:  C=2πr. We are given that C = 29.5. Thus we can plug in to get  [29.5]=2πr and then multiply 2π to get 29.5=(6.28)r.  Lastly, we divide both sides by 6.28 to get 4.70=r.   (The information given of 22 ounces is useless)

### Example Question #111 : Geometry

If the circumference of a circle is , what is the radius?      Explanation:

The formula for circumference is .

Plug in our given information.  Divide both sides by .  ### Example Question #112 : Geometry

Find the radius of a circle with area       Explanation:

Since the formula for the area of a triangle is plug in the given area and isolate for . This yields 13.

### Example Question #113 : Geometry

The circumference of a circle is 45 inches.  The circle's radius is ____ inches.      Explanation:

When you know the circumference of a circle, you can determine its diameter by dividing the circumference by .  Then, when you have the diameter, you can determine the radius by dividing the diameter by 2.

### Example Question #8 : How To Find The Length Of A Radius

A circle with center (8, 5) is tangent to the y-axis in the standard (x,y) coordinate plane. What is the radius of this circle?

5

8

16

4

8

Explanation:

For the circle to be tangent to the y-axis, it must have its outer edge on the axis. The center is 8 units from the edge.

### Example Question #10 : How To Find The Length Of A Radius

A circle has an area of . What is the radius of the circle, in inches?

49 inches

16 inches

24.5 inches

7 inches

14 inches

7 inches

Explanation:

We know that the formula for the area of a circle is πr2. Therefore, we must set 49π equal to this formula to solve for the radius of the circle.

49π = πr2

49 = r2

7 = r

### Example Question #1 : How To Find Circumference

A circle has radius . What is the circumference, rounded to the nearest tenth?      Explanation:

Circumference is given by the equation . We can use this equation with the given radius, 4.2, to solve for the circumference. ### Example Question #2 : How To Find Circumference

What is the circumference of a circle with a radius of 12?

What is the circumference of a circle with a radius of 12?     Explanation:

To find the circumference of a circle given the radius we must first know the equation for the circumference of a circle which is We then plug in the number for the radius into the equation yielding We multiply to find the value for the circumference is .

The answer is .

### Example Question #3 : How To Find Circumference

A circle with an area of 13π in2 is centered at point C. What is the circumference of this circle?

√13π

26π

2√13π

13π

√26π

2√13π

Explanation:

The formula for the area of a circle is πr2.

We are given the area, and by substitution we know that 13π πr2.

We divide out the π and are left with 13 = r2.

We take the square root of r to find that r = √13.

We find the circumference of the circle with the formula = 2πr.

We then plug in our values to find = 2√13π.

### Example Question #4 : How To Find Circumference

A 6 by 8 rectangle is inscribed in a circle. What is the circumference of the circle?

8π

6π

25π

10π

12π 