Use your string to estimate the length of the circumference of each circle in Math about Circles. Use a ruler to measure the string.

- Small circle circumference: ______
- Medium circle circumference: _____
- Large circle circumference: _____

Estimate the relationship between the circumference and the diameter for each of the three circles:

- The circumference is approximately ______ times the diameter.

Is that true for any circle? _____ Talk about it: Why do you think so?

AnswersThere is a special number that always relates the circumference of a circle and its diameter. You estimated it when you measured the circles with string. The number is π (say “pi” with a long i).

- Fill in C (for circumference) and d (for diameter) in the following formula: ___= π•___

π is an irrational number. That means that its digits to the right of the decimal point never form a repeating pattern. We can use estimates of π when finding the circumference and area of a circle.

Two common estimates for π are 3.14 and 22/7. But neither is exactly π.

This is π with many digits. Notice how they do not form a repeating pattern. (How can you check?) There are infinitely many more digits of π.