Exact Numbers

What make a number exact?

Exact Numbers

There are three different ways to know if a number is exact.

1. Numbers from counting things.2. Numbers used in unit conversions.3. Numbers in formulas.

Counting things

• When you count a whole number of things, you have an exact number.– When you count things like pennies you

have an exact number of pennies. – If you are counting pies and someone ate

half of one. You no longer have an exact number of pies.

Conversion of units

• When you use a conversion factor to convert from one unit to another, these are exact numbers.

If you have 2000 mL of milk and you know there are 1000mL in a 1L.

2000 mL X 1 L = 2L

1000 mL

Conversion factor exact numbers

Numbers in formulas

• When you use rational numbers in a formula , you have exact numbers

• Area of a triangle = ½bh the ½ is exact.

• Circumference of a square = 4s the 4 is exact

Exceptions in formulas

• When the formula includes an irrational number, the irrational number is not exact.

• circumference of a circle = 2πr the 2 is exact

π is not exact

Summary of Exact Numbers

• When you count a whole number of things, you have an exact number.

• When you use a conversion factor, these are exact numbers.

• When you use rational numbers in a formula , you have exact numbers.