Circle = Flat Round Shape with All Rim Points Equidistant from Center
Basics
r = Radius = distance from rim to center
d = Diameter = distance all the way across circle through its center
C = Circumference = distance around rim
A = Area = area covering surface of circle (measured in square units)
A circle occupies 360°.
π = pi
π = approximately 3.141592654 = approximately 3.14
π = ratio of a circle’s circumference to its diameter
π is primarily a multiplier.
When π is immediately adjacent to a number, letter, or parenthesis, an invisible multiplication sign is in force.
2π= 2 × π
πd = π × d
2(π) = 2 × π
π(d) = π × d
Formulas (Mathematics Formula Sheet)
C = 2πr
C = πd
A = πr²
Formulas (NOT on Mathematics Formula Sheet)
d = 2r
r =
d =
r =
r =
Question
A circle has a diameter of 18 centimeters. What is its radius?
Answer
9 centimeters
Answer Process
r =
r = = 9
Input  Display  Comment 
blinker  clears screen  
1 2 x 18  *18  r = 
9  Answer  
Circle 
Question
A circle has a radius of 13 centimeters. What is its diameter?
Answer
26 centimeters
Answer Process
d = 2r
d = 2 × 13 = 26
Input  Display  Comment 
blinker  clears screen  
2 × 13  2*13  d = 2r 
26  Answer  
Circle 
Question
A coin has a radius of 7 centimeters. What is its circumference?
Answer
14π centimeters
Answer Process
C = 2πr
C = 2 × π × 7 = 14π
Input  Display  Comment 
blinker  clears screen  
2 × × 7  2*π*7  C = 2πr 
14π  Answer  
Circle 
Question
With reference to the question above, what is the circumference of the coin in decimal terms?
Answer
43.98229715
Answer Process
See Toggle.
14π ⇔ 43.98229715
Input  Display  Comment 
blinker  clears screen  
2 × × 7  2*π*7  C = 2πr 
14π  
43.98229715  Answer (Toggle) 

Circle 
Question
With reference to the question above, what is the circumference of the coin to the nearest tenth of a centimeter?
Answer
44.0
Answer Process
See Rounding.
43.98229715 → 44.0
Question
An earring has a diameter of 24 centimeters. What is its circumference?
Answer
24π centimeters
Answer Process
C = πd
C = π × 24 = 24π
Input  Display  Comment 
blinker  clears screen  
× 24  π*24  C = πd 
24π  Answer  
Circle 
Question
A coin has a circumference of 3π centimeters. What is its diameter?
Answer
3 centimeters
Answer Process
d =
d = = 3
Input  Display  Comment 
blinker  clears screen  
3  d =  
3  Answer  
Circle 
Question
A coin has a circumference of 3π centimeters. What is its radius in decimal terms?
Answer
1.5 centimeters
Answer Process
r =
r = = = 1.5 after toggle
Input  Display  Comment 
blinker  clears screen  
3 2  r =  
1.5  Answer (Toggle) 

Circle 
Question
A coin has a radius of 7 centimeters. What is its area?
Answer
A = 49π square centimeters
Answer Process
A = πr²
A = π × 7² = 49π
Input  Display  Comment 
blinker  clears screen  
× 7  π*7²  A = πr² 
49π  Answer  
Circle 
Question
A coin has an area of 64π square centimeters. What is its radius?
Answer
8 centimeters
Answer Process
r =
r = = 8
Input  Display  Comment 
blinker  clears screen  
64  r =  
8  Answer  
Circle 
Question
An earring has a diameter of 24 centimeters. What is its area?
Answer
144π square centimeters
Answer Process
r =
r = = 12
A = πr²
A = π × 12² = 144π
Input  Display  Comment 
blinker  clears screen  
1 2 x 24  *24  r = 
12  r  
× 12  π*12²  A = πr² 
144π  Answer  
Circle 
Practice – Questions
1. A circle has a diameter of 22 centimeters. What is its radius?
2. A circle has a radius of 14 centimeters. What is its diameter?
3. A pizza has a radius of 6 inches. What is its circumference?
4. With reference to Question 3, what is the circumference of the pizza in decimal terms?
5. With reference to Question 3, what is the circumference of the pizza rounded to the nearest hundredth of an inch?
6. A pizza has a diameter of 14 inches. What is its circumference?
7. A pizza has a circumference of 4π inches. What is its diameter?
8. A pizza has a circumference of 7π inches. What is its radius in decimal terms?
9. A pizza has a radius of 6 inches. What is its area?
10. A pizza has a diameter of 16 inches. What is its area?
Practice – Answers
1. 11 centimeters
2. 28 centimeters
3. 12π inches
4. 37.69911184 inches
5. 37.70 inches
6. 14π inches
7. 4 inches
8. 3.5 inches
9. 36π square inches
10. 64π square inches