1. Exact Numbers What make a number exact?

2. 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.

3. 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.

4. 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

5. 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

6. 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

7. 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.