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

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

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 ( )

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