Circles

Diameter - a line going across the circle, dividing it into two equal halves.

Radius - a line measuring from the middle of the circle to the outside; half of the diameter.

Circumference - the distance, or perimeter around a circle found by diameter x pi.

Area - the amount of square units inside a designated space.

Geometry Formulas

Key

c = circumference

d = diameter

pi = 3.14

r = radius

b = base

h = height

a = area

/ = divide by

x = multiply

Formulas

d x pi = c

2r = d

2r x pi = c

d/2 = r

c/pi/2 = r

c/pi = d

a = pi x r x r.

bh = a

Area of a triangle: bh/2

Dividing Decimals

When dividing a decimal number [(e.g.) 5.6], these rules apply.

Dividing by 10

-move the decimal one space left

Dividing by 100

-move the decimal two spaces left

Dividing by 1000

-move the decimal three spaces left

Dividing by 0.1

-add one zero to the number or move the decimal one space to the right

Dividing by 0.01

-add two zeros to the dividend or move the decimal two spaces to the right.

Dividing by 0.001

-add three zeros to the dividend or move the decimal three spaces to the right.

Integer Rules

These rules apply to any operation done with integers such as multiplication, addition, subtraction, and division. The product, sum, difference or quotient will still be the same, just positive or negative according to the notes below.

-(+) x (+) = (+). (e.g.) (+5) = (+7) = (+12).

- (+) x (-) = (-).

- (-) x (-) = (+).

Divisibility Rules

Divisible by 1:

-all numbers are divisible by one.

Divisible by 2:

-if the number is even

-if the number ends in an even number or zero.

Divisible by 3:

-If the sum of all digits in the number is divisible by three.

Divisible by 4:

-if the last two digits in a number are divisible by four.

Divisible by 5:

-if the number ends in zero or five.

Divisible by 6:

-if the number is divisible by both two and three.

Divisible by 8:

-if the last three digits are divisible by eight.

Divisible by 9:

-if the sum of all digits is divisible by nine.

Divisible by 10:

-if the number ends in zero.