1. An example of a formula is Area circle =  x r2
Today we are going to look at using formulae. You know heaps of formulae already

2. Using Formulae and Substitution
1. We never write the ‘x’ sign in so:
3a means 3 x a
Eg If a = 5 and b = -2 then calculate:
3a – 4b2
(2b + a)(4a + 6b) =
= 3 x 5 – 4 x (-2)2 = -1
(2x-2+5)(4x5+6x-2) = 8

3. Be careful when evaluating fractional algebraic expressions Example:
If x = 5 evaluate:
You must use brackets to tell your calculator that it is all of the top ÷ all of the bottom
x + 7
x + 1
5 + 7
5 + 1 ( ) = = 2 ( )

4. Often these sort of skills will be put in a practical context:
Example: A person’s BMI is calculated by:
BMI = m ÷ h2
where:
m = mass in kg h = height in m
Calculate the BMI of Sophie who weighs 55kg and is 160 cm tall
BMI = 55 ÷ 1.62 = 21.5
160cm = 1.6m