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What is the Sq ft of a circle?

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I have a 15 ft circle and need to figure out the sq ft if you add 4 ft from that circle what would the sqft be around that circle? 

Apr 8th, 2015

Not sure if 15 feet is the diameter or the radius.  Here it is both ways.

Circumference = 2 (pi) r, if diameter = 15 feet, then radius = 7.5

C = 2 (pi) (7.5) = 15 (pi) feet

If 15 feet is the radius, then:

C = 2 (pi) (15) = 30 (pi) feet

Now if you add 4 feet (again to the diameter or radius?) Below is all 4 ways of 15 and 7.5 being radius and additional 4 feet being diameter and radius. Remember diameter is 1/2 the radius in length. Be specific when you ask these kinds of questions.

C = 2 (pi) (7.5 + 2) = 2 (pi)( 9.5) = 19 (pi) feet, original radius is 7.5 feet, add 2 additional feet radius

C = 2 (pi) (15 + 2) = 2 (pi) (17) = 34 (pi) feet, original radius is 15 feet, add 2 additional feet radius

C = 2 (pi) (7.5 + 4) = 2 (pi) (11.5) = 23 (pi) feet, original radius 7.5 feet, add 4 additional feet radius

C = 2 (pi) (15 + 4) = 2 (pi) (19) = 38 (pi) feet, original radius 15 feet, add 4 additional feet radius.

Make sure you specify if the "15 ft circle" if the 15 refers to the diameter or radius.  As you can see it makes a difference.  Also, when you say you "add 4 ft", again are you adding the 4 feet to the radius or diameter.

I did it all four ways so you could see that the interpretation matters. 




Apr 8th, 2015

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