The circumference of a circle is 64 pi meters. Find the diameter of the circle.

Question

Need help with this problem. I looked it up to see how some people did it, and it came up the opposite, with area instead of circumference. 

Answer 1

c = 2pi*r = pi*d

 

64*pi = pi*d

 

d = 64*pi/pi

 

   = 64 meters

Answer 2

You need to know a formula for the circumference of a circle, for this problem.  

 

This is one of the formulas for Circumference:

 

C = 2pi * r

 

(in words that says:  Circumference equals 2pi times the radius)

 

If you plug in 64 pi for the circumference, you get:

 

64 pi = 2pi * r

 

How would you solve for r?

 

You can divide both sides of the equation by the number that r was multiplied by (2pi).

 

64 pi  = 2 pi * r

------     ----

2 pi       2 pi

 

In this problem that gets you a nice neat number:   r = 32 and the units are meters, just like the circumference (they're both lengths).

 

Then the last step is remembering that they asked you for the diameter, not the radius.   The radius is only half way across the circle (from a point on the edge of the circle to the center).  The diameter is the whole way across the circle (from a point on one edge of the circle through the center, and then to a point on the other edge of the circle. )

 

 

How do you go from half of "something", to the whole "something"?  Multiply by 2.

 

So, the answer is,  d = 64 meters