Consider using a circle's radius as the unit for measuring its circle's circumference. If you were to measure any circle's circumference, C , using its radius, r, as the unit of measure, the measurement would be . As the circle's radius and circumference vary together, the circle's circumference C is always times as large as the circle's radius, r . Determine if the following statement is true or false: Since a circle's circumference C is proportional to its radius r, it follows that C is changing at a constant rate of change with r . For any change in a circle's radius, r , the circle's circumference changes by Since a circle's diameter is 2 times as long as its radius, we can also define a circle's circumference in terms of its diameter, d , by writing C= . Think about the meaning of the formula for circumference in terms of diameter you wrote above. The circumference, C , of any circle is always times as large as its diameter, d . If we measure a circle's circumference using its diameter as the unit of measurement, the answer will always be: Since Cd=, we know that (or about 3.14)