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?


There 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. ​

many digits of pi
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