How to Calculate the Radius of a Circle

Step-by-Step Guide

1. 1

   Step 1: Write down the circumference formula.

   The formula is C=2πr{\displaystyle C=2\pi r}, where C{\displaystyle C} equals the circle’s circumference, and r{\displaystyle r} equals its radius.The symbol pi{\displaystyle pi} ("pi") is a special number, roughly equal to
   3.14.
   
   You can either use that estimate (3.14) in calculations, or use the pi{\displaystyle pi} symbol on a calculator.
2. 2

   Step 2: Solve for r. Use algebra to change the circumference formula until r (radius) is alone on one side of the equation: C=2πr{\displaystyle C=2\pi r}C2π=2πr2π{\displaystyle {\frac {C}{2\pi }}={\frac {2\pi r}{2\pi }}}C2π=r{\displaystyle {\frac {C}{2\pi }}=r}r=C2π{\displaystyle r={\frac {C}{2\pi }}}

   , Whenever a math problem tells you the circumference C of a circle, you can use this equation to find the radius r.
   
   Replace C in the equation with the circumference of the circle in your problem:
   For example, if the circumference is 15 centimeters, your formula will look like this: r=152π{\displaystyle r={\frac {15}{2\pi }}} centimeters. , Enter your result in a calculator with the π{\displaystyle \pi } button and round the result.
   
   If you don't have a calculator, calculate it by hand, using
   3.14 as a close estimate for π{\displaystyle \pi }.
   
   For example, r=152π={\displaystyle r={\frac {15}{2\pi }}=} about
   7.52∗3.14={\displaystyle {\frac {7.5}{2*3.14}}=} approximately
   2.39 centimeters.
3. 3

   Step 3: Plug the circumference into the formula.

4. 4

   Step 4: Round to a decimal answer.

Detailed Guide

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