The subject of a formula is the letter on its own on one side. In y = x + 7, y is the subject. To make x the subject, use inverse operations as if solving an equation: subtract 7 from both sides to get x = y − 7.
For A = ½bh, multiply both sides by 2 to get 2A = bh, then divide by b: h = 2A/b. Do the same operations to both sides, one step at a time, until the letter you want is alone.
Worked example
Make x the subject of y = 3x − 5.
- Add 5 to both sides: y + 5 = 3x.
- Divide both sides by 3.
Answer:x = (y + 5)/3
Key words
- subject
- the letter a formula is solved for
- formula
- a rule linking quantities
- rearrange
- change which letter is the subject
Practise this
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