Factorise first, then cancel common factors
What you will learn
Know the rule, then use it
These are the short notes. Read each one, then check you can use it in the worked example below.
Method
To simplify algebraic fractions: factorise numerator and denominator fully, then cancel common factors
Factorise the numerator using the difference of two squares
x2 − 9 = (x + 3)(x − 3)
Factorise the denominator
x2 − x − 6 = (x − 3)(x + 2)
Cancel the common factor (x − 3)
(x + 3)(x − 3) / (x − 3)(x + 2) = (x + 3)/(x + 2)
Watch out
Students try to cancel across addition signs, writing (x2 − 9)/(x2 − x − 6) and cancelling x2
Factorise first, then cancel common factors.
Multiply through by the denominators, checking excluded values.
Simplify (x2 − 9)/(x2 − x − 6).
Factorise the numerator using the difference of two squares: x2 − 9 = (x + 3)(x − 3).
Factorise the denominator: x2 − x − 6 = (x − 3)(x + 2). Pair that multiplies to −6 and adds to −1 is (−3, +2).
Cancel the common factor (x − 3): (x + 3)(x − 3) / (x − 3)(x + 2) = (x + 3)/(x + 2).
State any excluded values: x ≠ 3 (denominator of original would be zero), x ≠ −2 (denominator of result would be zero).
(x + 3)/(x + 2)
Build up to the hardest questions
Do them in order. If you miss a step, read the solution, then redo the question without looking.
WorkedreasoningSimplify (x2 − 9)/(x2 − x − 6).
4 marks4 minsalgebraic-fractions-workedShow solution
Simplify (x2 − 9)/(x2 − x − 6).
- 1.Factorise the numerator using the difference of two squares: x2 − 9 = (x + 3)(x − 3).
- 2.Factorise the denominator: x2 − x − 6 = (x − 3)(x + 2). Pair that multiplies to −6 and adds to −1 is (−3, +2).
- 3.Cancel the common factor (x − 3): (x + 3)(x − 3) / (x − 3)(x + 2) = (x + 3)/(x + 2).
- 4.State any excluded values: x ≠ 3 (denominator of original would be zero), x ≠ −2 (denominator of result would be zero).
(x + 3)/(x + 2)
- M1: factorise the numerator using the difference of two squares
- M1: factorise the denominator
- M1: cancel the common factor (x − 3)
- M1: state any excluded values
- A1: (x + 3)/(x + 2)
Students try to cancel across addition signs, writing (x2 − 9)/(x2 − x − 6) and cancelling x2.Cancelling only works for factors in multiplication.Factorising first is the only safe way to identify what can be cancelled.
DiagnosticrecallSimplify (x2 − 4)/(x + 2).
1 mark2 minsalgebraic-fractions-q1Show solution
Simplify (x2 − 4)/(x + 2).
- 1.Spot the skill: To simplify algebraic fractions: factorise numerator and denominator fully, then cancel common factors.
- 2.Use the factorise the numerator using the difference of two squares stage first, then factorise the denominator.
- 3.Keep the final answer visible: x − 2.
x − 2
- M1: use the correct to simplify algebraic fractions: factorise numerator and denominator fully, then cancel common factors.never cancel terms that are added or subtracted — only cancel factors (multiplied parts).for adding/subtracting: find a common denominator, convert each fraction, combine numerators.
- A1: x − 2
Students try to cancel across addition signs, writing (x2 − 9)/(x2 − x − 6) and cancelling x2.Cancelling only works for factors in multiplication.Factorising first is the only safe way to identify what can be cancelled.
EasyprocedureSimplify (2x2 + 3x)/(4x2 − x).
2 marks3 minsalgebraic-fractions-q2Show solution
Simplify (2x2 + 3x)/(4x2 − x).
- 1.Spot the skill: To simplify algebraic fractions: factorise numerator and denominator fully, then cancel common factors.
- 2.Use the factorise the denominator stage first, then cancel the common factor (x − 3).
- 3.Keep the final answer visible: (2x + 3)/(4x − 1).
(2x + 3)/(4x − 1)
- M1: use the correct to simplify algebraic fractions: factorise numerator and denominator fully, then cancel common factors.never cancel terms that are added or subtracted — only cancel factors (multiplied parts).for adding/subtracting: find a common denominator, convert each fraction, combine numerators.
- A1: (2x + 3)/(4x − 1)
Students try to cancel across addition signs, writing (x2 − 9)/(x2 − x − 6) and cancelling x2.Cancelling only works for factors in multiplication.Factorising first is the only safe way to identify what can be cancelled.
MediumreasoningAdd 1/(x + 1) + 2/(x − 1). Simplify fully.
3 marks4 minsalgebraic-fractions-q3Show solution
Add 1/(x + 1) + 2/(x − 1). Simplify fully.
- 1.Spot the skill: To simplify algebraic fractions: factorise numerator and denominator fully, then cancel common factors.
- 2.Use the cancel the common factor (x − 3) stage first, then state any excluded values.
- 3.Keep the final answer visible: (3x − 1)/((x + 1)(x − 1)).
(3x − 1)/((x + 1)(x − 1))
- M1: use the correct to simplify algebraic fractions: factorise numerator and denominator fully, then cancel common factors.never cancel terms that are added or subtracted — only cancel factors (multiplied parts).for adding/subtracting: find a common denominator, convert each fraction, combine numerators.
- A1: (3x − 1)/((x + 1)(x − 1))
Students try to cancel across addition signs, writing (x2 − 9)/(x2 − x − 6) and cancelling x2.Cancelling only works for factors in multiplication.Factorising first is the only safe way to identify what can be cancelled.
Hardproblem solvingSolve 3/(x − 2) = 5/(x + 1).
3 marks5 minsalgebraic-fractions-q4Show solution
Solve 3/(x − 2) = 5/(x + 1).
- 1.Spot the skill: To simplify algebraic fractions: factorise numerator and denominator fully, then cancel common factors.
- 2.Use the state any excluded values stage first, then factorise the numerator using the difference of two squares.
- 3.Keep the final answer visible: x = .
x =
- M1: use the correct to simplify algebraic fractions: factorise numerator and denominator fully, then cancel common factors.never cancel terms that are added or subtracted — only cancel factors (multiplied parts).for adding/subtracting: find a common denominator, convert each fraction, combine numerators.
- A1: x =
Students try to cancel across addition signs, writing (x2 − 9)/(x2 − x − 6) and cancelling x2.Cancelling only works for factors in multiplication.Factorising first is the only safe way to identify what can be cancelled.
Exam-stylemulti-stepSimplify (x2 + 5x + 6)/(x2 + 4x + 3).
4 marks6 minsalgebraic-fractions-q5Show solution
Simplify (x2 + 5x + 6)/(x2 + 4x + 3).
- 1.Spot the skill: To simplify algebraic fractions: factorise numerator and denominator fully, then cancel common factors.
- 2.Use the factorise the numerator using the difference of two squares stage first, then factorise the denominator.
- 3.Keep the final answer visible: (x + 2)/(x + 1).
(x + 2)/(x + 1)
- M1: use the correct to simplify algebraic fractions: factorise numerator and denominator fully, then cancel common factors.never cancel terms that are added or subtracted — only cancel factors (multiplied parts).for adding/subtracting: find a common denominator, convert each fraction, combine numerators.
- A1: (x + 2)/(x + 1)
Students try to cancel across addition signs, writing (x2 − 9)/(x2 − x − 6) and cancelling x2.Cancelling only works for factors in multiplication.Factorising first is the only safe way to identify what can be cancelled.
Grade 9 stretchproblem solvingSolve 3/(x + 1) = 2/(x - 2).
4 marks7 minsalg-frac-g9Show solution
Solve 3/(x + 1) = 2/(x - 2).
- 1.Cross multiply.
- 2.Expand and solve the linear equation.
- 3.Check excluded values.
x = 8
- M1: 3(x - 2) = 2(x + 1)
- A1: x = 8
Do not rush straight into arithmetic. Select the relevant method and show a complete chain of working.
Switch between skills
Set a timer and attempt all four questions before opening any answers. This is closer to the way skills appear in a real paper.
1Algebraic fractions - 2 marksSimplify (x2 − 4)/(x + 2).Mark answer
x − 2
2Algebraic notation and substitution - 2 marksFind t² + 4t when t = −2Mark answer
−4
3Simplifying expressions - 2 marksSimplify 2xy + 3x²y − xy + x²yMark answer
3x²y + xy
4Expanding and factorising - 3 marksFactorise fully 6x² + 9xMark answer
3x(2x + 3)
- I can explain the method for algebraic fractions.
- I can show clear working without skipping key steps.
- I can avoid this mistake: Students try to cancel across addition signs, writing (x2 − 9)/(x2 − x − 6) and cancelling x2.Cancelling only works for factors in multiplication.Factorising first is the only safe way to identify what can be cancelled.
This guide follows the Pearson Edexcel GCSE Mathematics 1MA1 specification. Practice questions are original Learnova questions shaped around official content and exam skills.