What is the Sq ft of a circle?
Mathematics

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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?
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.
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