How to change the subject of a formula. A formula is a special equation which deals with ‘variables.’
Variables are bits of information that can change each time you deal with the formula.
For instance, one of the best known formulas is:
Speed = Distance / Time … and we use it every time we go somewhere.
If you’re driving at 30 miles per hour – the numbers can be put in place of the words:
Speed = Distance (30 miles) / Time (every 1 hour)
Speed = 30 miles / hour
Exam questions, that are based around this equation, usually ask you to work out one of the ‘terms.’ The questions are usually along the lines of:
- How long does it take to travel 60 miles at a speed of 120 miles per hour?
- How far have I travelled if I walk at 4mph for 3 hours?
- there are only three variables and
- the three variables are all directly, or proportionately, related
Unfortunately it can be a little more difficult to change the subject (the first bit) to another part of the formula – particularly when there doesn’t seem to be a direct relationship.
Watch the videos and try the quick test.
Click here to try the Quick Test Rearranging Formula
Learning how to change the subject of a formula can be important depending upon what we are looking for; or different interpretations of the same thing. For instance in the UK we use degrees Centigrade to measure temperature, in America they use degrees Farenheit.
If you want to convert between Centigrade and Farenheit, both these formulas contain the same information – although each have different subjects:
Follow these steps when learning how to change the subject of a formula:
- Always work down the page with the equals sign in the middle
- Deal with terms in the correct order (use BIDMAS: Brackets, Index (sometimes Order), Division and Multiplication together, Addition and Subtraction together
- Always keep the formula in balance, write each side what you are doing
- Don’t lose track of what you are trying to achieve!
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