Area and Circumference Cheat Sheet

Area and Circumference Cheat Sheet

Before we even get started…

To find the radius, we must divide the diameter by two.

To find the diameter, you add the radius together twice.

Area Formula-- A =  x r2 (to square a number, you must multiply that number by itself 5 x5 = 25 NOT 5 x2= 10)

Circumference Formula-- C = x d (to get the diameter, you add the radius together twice) ***This is not to be confused with squaring a number*** If the radius of a circle is 10, then you add 10 +10 = 20.

Circumference Problem

The diameter of each wheel on this bicycle is 22 inches.

Which is closest to the circumference of the wheel?Use π = 3.14.
A. 34.54 inches / A
B. 69.08 inches
C. 1519.8 inches
D. 379.94 inches
(If the problem HAD GIVEN you the radius, you would add it
together twiceRadius=10then 10+10= 20, so 20 would be the diameter)
Step One- write the formula

C = x d

Step Two- plug in the numbers for each of the letters one at a time.

C = x d

C = 3.14 x d

C = 3.14 x 22

Step Three- perform the math

3.14

x 22

69.08 in.

Area of a Circle Problem

The diameter of a circle is 6.4 cm.Which is the closest to the area of the circle?
A. 20.10 cm2
B. 128.16 cm2
C. 10.05 cm2
D. 32.15 cm2
Because the diameter is given instead of the radius, you must divide the diameter by two.
A = x r2 1. Formula written
A = 3.14 x r2 2. We write 3.14 instead of 
A = 3.14 x 3.22 3. We write in the radius
A = 3.14 x 10.24 4. We square the radius
A = 32.15 cm2 5. We have multiplied 3.14 x 10.24 to get 32.15cm2