© boardworks ltd 20141 of 7 a8 changing the subject and deriving formulae this icon indicates the...
DESCRIPTION
© Boardworks Ltd of 25 © Boardworks Ltd of 7 The subject of a formula Look at the formula, V = IR where V is voltage, I is current and R is resistance. V is called the subject of the formula. The subject of a formula always appears in front of the equals sign without any other numbers or operations. Sometimes it is useful to rearrange a formula so that one of the other variables is the subject of the formula. Suppose, for example, that we want to make I the subject of the formula V = IR.TRANSCRIPT
© Boardworks Ltd 20141 of 7
A8 Changing the subject and deriving formulae
This icon indicates the slide contains activities created in Flash. These activities are not editable.
For more detailed instructions, see the Getting Started presentation.
© Boardworks Ltd 20141 of 7
© Boardworks Ltd 20092 of 25 © Boardworks Ltd 20142 of 7
Using inverse operations
Andy is 5 years older than his brother, Brian. Their ages are linked by the formula:
where A is Andy’s age in years and B is Brian’s age in years.Using this formula it is easy to find Andy’s age given Brian’s age.Suppose we want to find Brian’s age given Andy’s age.Using inverse operations, we can write this formula as:
A = B + 5
B = A – 5
© Boardworks Ltd 20093 of 25 © Boardworks Ltd 20143 of 7
The subject of a formula
Look at the formula,
V = IR
where V is voltage, I is current and R is resistance.
V is called the subject of the formula.
The subject of a formula always appears in front of the equals sign without any other numbers or operations.
Sometimes it is useful to rearrange a formula so that one of the other variables is the subject of the formula.
Suppose, for example, that we want to make I the subject of the formula V = IR.
© Boardworks Ltd 20094 of 25 © Boardworks Ltd 20144 of 7
The formula:
can be written as:
I × R V
The inverse of this is:
V÷ RI
or
Changing the subject of the formula
V is the subject of this formula
I is now the subject of this formulaI = V
R
V = IR
© Boardworks Ltd 20095 of 25 © Boardworks Ltd 20145 of 7
Matchstick pattern
Look at this pattern made from matchsticks:
PatternNumber, n 1Number ofMatches, m 3
2
5
3
7
4
9
The formula for the number of matches, m, in pattern number n is given by the formula:
m = 2n + 1
Which pattern number will contain 47 matches?
© Boardworks Ltd 20096 of 25 © Boardworks Ltd 20146 of 7
The formula:m = 2n + 1
can be written as:
n + 1× 2 m
The inverse of this is:
m÷ 2 – 1n
or
m is the subject of this formula
n is the subject of this formula
Changing the subject of the formula
n = m – 12
© Boardworks Ltd 20097 of 25 © Boardworks Ltd 20147 of 7
Equivalent formulae