Using Formulae• We use formulae all the time.• The algebraic equation just

lets us put in numbers as we find them.

• Formulae such as;

• A = πr2 (area of a circle)• A = ½ bh (area of a triangle)• V = lbh (volume of a cuboid)

• I know that we have already looked at this earlier BUT…..

• IT IS ESSENTIAL that you know how to use such formulae and to be able to manipulate them.

• In many forms of employment, similar formulae are used and you will be expected to be able to use and apply them.

Write a Simple Formula• Look at the examples 1 and 2 on page 36

of the small textbook.• Make sure you understand how the

formulae are produced.• We will now try an example;David is h cm tall. Sue is 12 cm taller than

David. Write down an expression for Sue’s height in terms of h.

• If unsure, I will do this on the left board.

Substitution• This is no more than replacing letters with

the appropriate numbers provided in the question.

• Look at example 3 on page 36 in the text book.

• You replace the letters with the appropriate given numbers and then do the calculation (CAREFULLY).

Now try to do the following example:H = 3(4x – y), find the value of H when

x = 5 and y = 7.1. Replace the letter x with 5

3(4 x 5 - y)2. Replace the letter y with 7

3(4 x 5 - 7)3. Do the calculation

You should get …… 39

Now try some from the small textbook p 36/7

All odd numbered questions up to question 15

Rearranging FormulaeSometimes the formula given is such

that the ‘bit’ to work out is in the main body of the equation, not on its own on the left side.

When this occurs we need to be able to rearrange the equation so that the ‘bit’ we want is on its own on the left hand side.

Look at this equationP = 2(L + B) (the perimeter of a rectangle)

We may be asked to make L the subject. This means that ‘L’ must be on its own on the left hand side.

Remember the rule:

Change side, change signUsing this rule move the

‘unwanted bits’ to the left side of the equals sign.

P = 2 (L + B)1. Remove the “2” (becomes a “ 2”

2. Remove the “B” (becomes a “–B”)

3.Finally swap the whole equation round

BLp 2

LBp 2

BP

L 2