ACT Math : Radius
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Example Questions
Example Question #2 : How To Find Circumference
A 6 by 8 rectangle is inscribed in a circle. What is the circumference of the circle?
8π
25π
10π
12π
6π
10π
First you must draw the diagram. The diagonal of the rectangle is also the diameter of the circle. The diagonal is the hypotenuse of a multiple of 2 of a 3,4,5 triangle, and therefore is 10.
Circumference = π * diameter = 10π.
Example Question #2 : How To Find Circumference
A gardener wants to build a fence around their garden shown below. How many feet of fencing will they need, if the length of the rectangular side is 12 and the width is 8?
4π + 24
40 ft.
96 ft
8π + 24
8π + 24
The shape of the garden consists of a rectangle and two semi-circles. Since they are building a fence we need to find the perimeter. The perimeter of the length of the rectangle is 24. The perimeter or circumference of the circle can be found using the equation C=2π(r), where r= the radius of the circle. Since we have two semi-circles we can find the circumference of one whole circle with a radius of 4, which would be 8π.
Example Question #3 : How To Find Circumference
The diameter of a circle is defined by the two points (2,5) and (4,6). What is the circumference of this circle?
π√5
2.5π
None of the other answers
π√2.5
5π
π√5
We first must calculate the distance between these two points. Recall that the distance formula is:√((x2 - x1)2 + (y2 - y1)2)
For us, it is therefore: √((4 - 2)2 + (6 - 5)2) = √((2)2 + (1)2) = √(4 + 1) = √5
If d = √5, the circumference of our circle is πd, or π√5.
Example Question #1 : How To Find Circumference
A car tire has a radius of 18 inches. When the tire has made 200 revolutions, how far has the car gone in feet?
500π
3600π
300π
600π
600π
If the radius is 18 inches, the diameter is 3 feet. The circumference of the tire is therefore 3π by C=d(π). After 200 revolutions, the tire and car have gone 3π x 200 = 600π feet.
Example Question #46 : Radius
A circle has the equation below. What is the circumference of the circle?
(x – 2)2 + (y + 3)2 = 9
The radius is 3. Yielding a circumference of 
Example Question #581 : Plane Geometry
What is the circumference of a cirle with a radius of seven?
Leave your answer in terms of 
Plug the radius into the circumference formula:
Example Question #584 : Act Math
A circle has an area of 
To find the circumference of a circle we use the formula
In order to solve we must use the given area to find the radius. Area of a circle has a formula of
So we manipulate that formula to solve for the radius.
Then we plug in our given area.
Now we plug our radius into the circumference equation to get the final answer.
Example Question #2 : How To Find Circumference
A circle has an area of 
To find the circumference of a circle we use the formula
In order to solve we must use the given area to find the radius. Area of a circle has a formula of
So we manipulate that formula to solve for the radius.
Then we plug in our given area.
Now we plug our radius into the circumference equation to get the final answer.
Example Question #581 : Geometry
What is the circumference of a circle with a radius of six ft? Leave your answer in terms of 
To find the circumference given the radius, simply double the radius and mulitply by 
Thus:
Example Question #114 : Circles
A running track can be formed by adding a semicircle to each of the short ends of a rectangle of dimension 
If Tracy runs one lap around the track described above, how far has she run?
The questions really asks us to find the perimeter of the track.This would be the two long sides of the rectangle plus the circumferences of each of the two semicircles.
Each long side of the triangle is 
Each semicircle has a circumference that is 
Since the diameter of the semicircle is equal to the width of the rectangle, 
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