Year 9 Interactive Maths - Second Edition

Rearrangement of Formulas

A = lw

for the area, A, of a rectangle of length l and width w.

The formula describes A in terms of other variables (i.e. pronumerals) and we can find the value of A simply by substituting the values of l and w.  We say that A is the subject of the formula.

To obtain the subject of the formula, we use the following rules:

• The same number can be added to each side of a formula.
• The same number can be subtracted from each side of a formula.
• Both sides of a formula can be divided by the same number.
• Both sides of a formula can be multiplied by the same number.

Literal Equations

The methods used to transpose literal equations are the same as those for solving equations.

Transpositions Involving One Operation

To rearrange the terms in a formula, the same operations are performed on both sides of the formula, i.e. whatever is done to one side of the formula must be done to the other.

Transpositions Involving Addition

Remember that we can add the same number to both sides of a formula.

Example 34

Transpose the formula I = A – P to make A the subject.

Solution:

Key Terms

subject of a formula, literal equation