How to Find the Area in Diameter & Then Rounding

Use pi to analyze a pie.

There's pie and there's pi. One is a delicious, circular dessert; the other represents the constant ratio between the circumference of a circle and its diameter. Because of this relationship, you can use pi to discover the amount, or area, of your pie. Pi will tell you exactly how much of that cherry pie on the table is waiting to be eaten. A few simple calculations and rounding will give you the area represented in a circle's diameter.

Things You'll Need

• Measuring stick
• Scientific calculator

Instructions

1. Measure the width from side to side, or diameter, of the circle you wish to analyze using a measuring stick. For example, you find the diameter of a cherry pie is 20.1 cm.

2. Divide this number by two using a calculator. For example, 20.1/2 = 10.05.

3. Square your answer using a calculator. For example, 10.05 x 10.05 = 101.

4. Multiply the answer by pi using a calculator. Pi is approximately equal to 3.14. For example, 101 x 3.14 = 317.14.

5. Use as many digits in your answer as there are in the number with the fewest digits that was used to calculate your answer. For example, both of the numbers you used to calculate your answer, 101 and 3.14, have three significant digits, so your final answer should have three digits.

6. Count this number of digits from the left of your answer. For example, you count to the 4 in 314.17.

7. Leave this number unchanged and drop all the digits after it if the number after it is less than five. For example, the number after 4 in 314.17 is 1. One is less than five, so you drop all the digits after 4 and leave the 4 unchanged. The area of the cherry pie is 314 square centimeters.

8. Add one to this number and drop all the digits after it if the number after it is greater than or equal to five. For example, the number after 4 in 314.57 is 5. The number 5 is equal to five, so you drop all the digits after 4 and add one to the 4. Your answer in this second case would be is 315 square centimeters.

References

• University of St. Andrews, Scotland: A History of Pi
• West Texas A&M University: Beginning Algebra: Tutorial 32, Formulas
• City University of New York: Rounding Off Numbers
• Purdue University: Significant Figures