2 USING FORMULAE

Formulae is the

plural of formula

It is not only areas that can be worked out with simple formulae.

eg.1. Peter delivers newspapers on six days a week. How much is he paid in a week if he is paid £2 per day?

eg.2. What is the mass of a piece of copper which has a density of 8.96 and a volume of 24cm2?

eg.3. Work out the area of a parallelogram that is 9cm long and has a height of 6cm.
eg.1. P = rd

P = 2x6

P = £12

eg.2.

 M = dv

M = 8·96x24

M = 215·04g

eg.3.

 A = lh

A = 9x6

A = 54 cm2

Notice how the answers are set out. If you haven’t already, get into the habit of always setting out your working clearly, even on the most simple answers. This will remove many of the obstacles that could arise when the problems become more complex.

2A Work out the following using the formula given. It is not necessary to know what the formulae are for. The algebra tells us what to do. That is what makes the algebra so useful and so powerful.

1. F=ma when m=12, a=9               7. A=lh when l=18, h=17

2. d=st when s=5, t=6                      8. W=fs when f=7, s=15

3. C=p d when d=10                      9. T=Fr when F=8.5, r=2

4. E=mg when m=20, g=9.8         10. M=mv when m=23, v=3.5

5. D=px when p=25, x=1.6           11. e=ul when u=2, l=0.003

6. v=at when a=5.5, t=15             12 V=mp when m=0.49, p=361

The formula A = lw allows you to work out the area of a rectangle when you have its length and its width.

But it also allows you to work out its width if you have its area and its length.

eg. Find the width of the rectangle that has a length of 12cm and an area

of 132cm2.
Write down the formula

Substitute the given values

Do the calculation

 A = lw

132 = 12w

11 = w

 This says that 12 times a number is 132, so to work out the number, divide by 12

2B

1. Use the formua A=lw to find w when:-

(a) A=20, l=5 (c) A=65. l =15 (e) A=32.5, l =6.5

(b) A=50, l =12 (d) A=24.5, l =7

2. Use the fomula A=lw to find l when:-

(a) A=44, w=4 (c) A=65, w =8 (e) A=50, w =7.07

(b) A=50, w =6 (d) A=32.5, w =5

2C Exactly the same method is used to solve these equations.

Although you may recognise some of the formulae, again you do not

need to know what they are for.

  1. A=lh find l when A=50, h =8
  2. F=ma find a when F=90, m=12
  3. a=bh find b when a=72, h=9
  4. d=st find s when t=3.5, d=15
  5. C=p d find d when C=125
  6. P=4l find l when P=30

   7.  h=ac find a when h=12, c=0.5

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