The symbol π{\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 π{\displaystyle \pi } symbol on a calculator.
If you’re not sure where the circle center is, put the ruler down across your best guess. Hold the zero mark of the ruler steady against the circle, and slowly move the other end back and forth around the circle’s edge. The highest measurement you can find is the diameter. For example, you might have a circle with a diameter of 4 centimeters.
For example, if the diameter is 4 cm, the radius equals 4 cm ÷ 2 = 2 cm. In math formulas, the radius is r and the diameter is d. You might see this step in your textbook as r=d2{\displaystyle r={\frac {d}{2}}}.
