Courtney T.

asked • 01/09/13

I need help on finding the Circumference and Area of Circles.

I  need help on this, my math teacher is not a very good teacher when it comes to teaching. Please explain this to me. This > π < is the pi symbol. Thanks!

Solve using:        C = 2πr = πd   or      A = πr2

  1. What is the circumference of a circle with a radius of 6?
  2. The circumference of a circle is 24π. What is the radius?
  3. If the circumference of a circle is 32π, what is the area?
  4. What us the area of a circle with a radius of 5?
  5. The area of a circle is 169π. What is the radius?
  6. The area of a circle is 100π. If the radius is doubled, what is the new area?


Bill F.

Hey Courtney... Apparently the Answer section below isn't working...

p is just a definition: it's the ratio between a circle's diameter and its circumference.  In other words, any circle circumference is approximately 3.14 (p) times longer (around) than its diameter length.  Some old Greek dude discovered it a few thousand years ago...  Let's do the first 3 problems together - by then I think you'll have confidence you can do the remaining 3!

1.  If C = 2pr, then we substitute C = 2*3.14*6, = 37.68

2.  If C = 2pr, then let's rearrange that so we can find r:  divide both sides (to keep it equal) by 2p to get r by itself: C/2p = 2pr/2p, so r = C/2p.  Now substitute:  r = 24p/2p = 12

3.  If C = 2pr, then r = C/2p  (see #2 above), then r = d/2.  

If A = pr2, then A = p (C/2p)2.  Substitute:  A = p (32p/2p)2, or A = p (16)2

A = 3.14 * 256 = 803.84

Now can you complete #3 - #6?  I bet you can...   :-)



Bill F.

I tried adding answers via Answers below, and also via Comments - no go!




Amazing! I was 10 yrs. old in Mexico and learned that Circumference = Diameter times pi(3.14).  So, if you have a circle with a diameter (from edge to edge) of 5 in. you X 3.14 =15.7 in. diameter. 
Now to find the area of a circle, you square (multiply the # by itself) X the radius  (from center to edge).  
So, as in the 1st. example the diameter was 5 in. the radius if 2.5 in. so 2.5 X 2.5= 6.25 in. X 3.14= 19.625 in.


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