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How Do You Find the Radius of a Circle if You Know the Circumference?

The circumference of the circle is 18.84 inches. Find the radius. Use 3.14 for π.

Summary

  1. Here, 'in.' is an abbreviation of 'inches'
  2. 'c' is the circumference
  3. 'π' is pi, an irrational number
  4. 'd' is the diameter
  5. 'r' is the radius
  6. There are two formulas that we can use for the circumference of a circle
  7. We want to use c=2πr because it contains 'r', which we're solving for
  8. We plugged our information into the c=2πr formula
  9. So we get '3 = r', which means the radius is 3 inches!

Notes

    1. There are two formulas that can be used for the circumference of a circle:
    2. c = πd and c = 2πr
    3. Since we want to solve for the radius, let's use c = 2πr
    1. The circumference, 'c', is 18.84 inches
    2. Since 'π' is an irrational number, we use its approximation of 3.14
    3. We are trying to solve for 'r', which is the radius
    1. The circumference, 'c', is 18.84 inches
    2. Since 'π' is an irrational number, we use its approximation of 3.14
    3. We are trying to solve for 'r', which is the radius
    4. Plugging in, we get:
    5. 18.84 = 2(3.14)r
    1. Since 'π' is an irrational number, we use its approximation of 3.14
    2. Notice there are the same amount of decimal places (two!) in the answer as the combined amount of spaces in the numbers being multiplied!
    1. 18.84 = 6.28r
    2. The goal is to isolate and solve for 'r', which is the radius
    3. 18.84/6.28 = 3
    4. So we get "3 = r", which means the radius is 3 inches!