1. Calculate the length of the radius. If you only know (or have measured) the diameter (the distance from one side of the circle to the other), then divide it by two to obtain the radius. The radius of a normal circle is always one-half of the diameter. 2. Read the formula. The formula for finding the area of a circle is
Area equals pi multiplied by (r squared)
3. Multiply the radius by itself to square it. For example, if your radius was 6 cm, your radius squared would be 36 cm squared. 4. Multiply the result you get from Step 3 by pi. (Use this also in "Finding the Area of a Sector"). * If the instructions say "leave in terms of pi" or "exactly", then just stick the pi onto your answer (e.g., the area of the circle = π36 cm^2.) If you just round it to 3.14, it's not at all exact. There is no way to show it exactly besides leaving the pi sign. * If the instructions say anything about rounding, replace pi with 3.14 or use your calculator's pi button.
Finding the Area of a Sector
1. Find out how big the sector is in terms of degrees. Unfortunately, there is no set way to do this. It will vary considerably, depending on what information is being supplied in the problem, and it is not possible to include a step-by-step process for every situation. 2. Figure out the radius of the circle. Again, the radius is exactly one half of the diameter. 3. Find the area of the circle. See the above section for instructions on how to do this. 4. Create a fraction
* The numerator as the sector's central angle in degrees, and * 360° as the denominator.
5. Simplify the fraction down to lowest terms. Find the lowest common denominator to simplify your fraction. 6. Multiply that fraction by the area of the circle. Your are finished! Special circles
1. Know what to do on these special cases:
* Occasionally you will see "a circle inside of a square". The side length of that square is equal to...