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Home > Maths II > Relationships > Changing the subject of a formula

Maths II

Changing the subject of a formula

Sometimes we will need to rearrange a formula to find the value of a subject.

For instance, we may know the area of a circle and need to find the radius. To do this, rearrange the formula to make the radius the subject.

# Changing the subject of a formula

The area of a circle (A) is

So

If we know the radius, it is easy to find the area using this formula. However, if we know the area and want to find the radius, rearrange the formula to make 'r' the subject.

[Start, by dividing both sides by ]

[Then take the square root of both sides]

or

Question

The formula connecting ºC and ºF is -

Rearrange the formula to make 'F' the subject.

[Multiply by 9]

[Divide by 5]

So F =

Note: is also correct.

Question

The formula for the volume (V) of a sphere is

Rearrange the formula to make 'r' the subject.