Here we will answer the following three questions. a) What is 21Pi? (What is 21π?), b) What is the area of a circle if the radius is 21?, and c) What is the circumference of circle if the radius is 21?
21π is another way of writing 21 times Pi (21 x 3.14). Therefore, the answer to our first question is:
21π = 65.97
Area: Next, we will calculate the area of a circle if the radius is 21. The formula for the area of a circle is πr2, which means that the area of a circle with a radius of 21 is:
π212 = 1385.44
Circumference: Finally, we will calculate the circumference of a circle if the radius is 21. The formula for the circumference of a circle is 2πr, which means that the circumference of a circle with a radius of 21 is:
2π21 = 131.95
Note: π or Pi has an infinite number of decimals. For our calculations on this page with a radius of 21, we used Pi with eleven decimals as 3.14159265358 and then we rounded all our answers to two decimals.
Helpful Definitions:
Radius: The distance between the center of a circle to the edge of the circle. In our example on this page it was 21.
Diameter: The distance between one side of a circle to the exact opposite side of the circle. This is the same as 2 times radius. (For example, if the radius is 21, then the diameter must be 42.)
Pi: The ratio of a circle's circumference to its diameter. This is a fixed number regardless of circle size.
We used the following formulas on this page, where A is Area of circle, C is Circumference of circle, and r is radius:
A = πr2
C = 2πr
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