Multiplying & Dividing Algebraic Fractions (Edexcel IGCSE Maths A): Revision Note

Exam code: 4MA1

Multiplying & dividing algebraic fractions

How do I multiply algebraic fractions?

  • STEP 1

    Simplify both fractions first by fully factorising

    • E.g. fraction numerator x over denominator 3 x plus 6 end fraction cross times fraction numerator 2 x plus 4 over denominator x plus 7 end fraction equals fraction numerator x over denominator 3 open parentheses x plus 2 close parentheses end fraction cross times fraction numerator 2 open parentheses x plus 2 close parentheses over denominator x plus 7 end fraction

  • STEP 2

    Cancel any common factors on top and bottom (from either fraction)

    • E.g. fraction numerator x over denominator 3 up diagonal strike open parentheses x plus 2 close parentheses end strike end fraction cross times fraction numerator 2 up diagonal strike open parentheses x plus 2 close parentheses end strike over denominator x plus 7 end fraction equals x over 3 cross times fraction numerator 2 over denominator x plus 7 end fraction

  • STEP 3
    Multiply the tops together
    Multiply the bottoms together

    • E.g. fraction numerator 2 x over denominator 3 open parentheses x plus 7 close parentheses end fraction

  • STEP 4

    Check for any further factorising and cancelling

    • E.g. fraction numerator 2 x over denominator 3 open parentheses x plus 7 close parentheses end fraction has no common factors so is in its simplest form

Worked Example

Write the following as a single fraction in its simplest form

fraction numerator 2 x squared minus 11 x plus 12 over denominator x squared plus 3 x minus 28 end fraction cross times fraction numerator x squared minus 5 x minus 14 over denominator 9 minus 4 x squared end fraction

Answer:

Factorise the four quadratics

2 x squared minus 11 x plus 12

  • Two numbers that multiply to give 2×12=24 and add to give -11

  • -3 and -8

2 x squared minus 3 x minus 8 x plus 12
x open parentheses 2 x minus 3 close parentheses minus 4 open parentheses 2 x minus 3 close parentheses
open parentheses 2 x minus 3 close parentheses open parentheses x minus 4 close parentheses

x squared plus 3 x minus 28

  • Two numbers that multiply to give -28 and add to give 3

  • 7 and -4

open parentheses x plus 7 close parentheses open parentheses x minus 4 close parentheses

x squared minus 5 x minus 14

  • Two numbers that multiply to give -14 and add to give -5

  • 2 and -7

open parentheses x plus 2 close parentheses open parentheses x minus 7 close parentheses

9 minus 4 x squared

  • Difference of two squares

  • 3 squared minus open parentheses 2 x close parentheses squared

open parentheses 3 minus 2 x close parentheses open parentheses 3 plus 2 x close parentheses

Write the expression using the factors

fraction numerator open parentheses 2 x minus 3 close parentheses open parentheses x minus 4 close parentheses over denominator open parentheses x plus 7 close parentheses open parentheses x minus 4 close parentheses end fraction cross times fraction numerator open parentheses x plus 2 close parentheses open parentheses x minus 7 close parentheses over denominator open parentheses 3 minus 2 x close parentheses open parentheses 3 plus 2 x close parentheses end fraction

Rewrite 2 x minus 3 as negative open parentheses 3 minus 2 x close parentheses and cancel common factors

fraction numerator negative up diagonal strike open parentheses 3 minus 2 x close parentheses end strike up diagonal strike open parentheses x minus 4 close parentheses end strike over denominator open parentheses x plus 7 close parentheses up diagonal strike open parentheses x minus 4 close parentheses end strike end fraction cross times fraction numerator open parentheses x plus 2 close parentheses open parentheses x minus 7 close parentheses over denominator up diagonal strike open parentheses 3 minus 2 x close parentheses end strike open parentheses 3 plus 2 x close parentheses end fraction
fraction numerator negative 1 over denominator open parentheses x plus 7 close parentheses end fraction cross times fraction numerator open parentheses x plus 2 close parentheses open parentheses x minus 7 close parentheses over denominator open parentheses 3 plus 2 x close parentheses end fraction

Write as a single fraction

  • open parentheses x plus 7 close parentheses and open parentheses x minus 7 close parentheses are not common factors

  • You can multiply open parentheses x minus 7 close parentheses by -1 to get rid of the negative sign in front of the front

  • Leave the expressions factorised

fraction numerator negative open parentheses x plus 2 close parentheses open parentheses x minus 7 close parentheses over denominator open parentheses x plus 7 close parentheses open parentheses 3 plus 2 x close parentheses end fraction space or space fraction numerator open parentheses x plus 2 close parentheses open parentheses 7 minus x close parentheses over denominator open parentheses x plus 7 close parentheses open parentheses 3 plus 2 x close parentheses end fraction

How do I divide algebraic fractions?

  • Flip (find the reciprocal of) the second fraction and replace ÷ with ×

    • So divided by a over b becomes cross times b over a

    • E.g. fraction numerator 3 x minus 12 over denominator x end fraction divided by fraction numerator 2 x plus 8 over denominator x plus 3 end fraction equals fraction numerator 3 x minus 12 over denominator x end fraction cross times fraction numerator x plus 3 over denominator 2 x plus 8 end fraction

  • Then follow the same rules for multiplying two fractions

Worked Example

Divide fraction numerator x plus 3 over denominator x minus 4 end fraction by fraction numerator 2 x plus 6 over denominator x squared minus 16 end fraction, giving your answer as a simplified fraction.

Answer:

Division is the same as multiplying by the reciprocal (the fraction flipped)

fraction numerator x plus 3 over denominator x minus 4 end fraction divided by fraction numerator 2 x plus 6 over denominator x squared minus 16 end fraction equals fraction numerator x plus 3 over denominator x minus 4 end fraction cross times fraction numerator x squared minus 16 over denominator 2 x plus 6 end fraction

Factorise all numerators and denominators to see which factors cancel out
You need to use the difference of two squares, x squared minus 4 squared equals open parentheses x minus 4 close parentheses open parentheses x plus 4 close parentheses

fraction numerator x plus 3 over denominator x minus 4 end fraction cross times fraction numerator x squared minus 16 over denominator 2 x plus 6 end fraction equals fraction numerator up diagonal strike x plus 3 end strike over denominator up diagonal strike x minus 4 end strike end fraction cross times fraction numerator up diagonal strike open parentheses x minus 4 close parentheses end strike open parentheses x plus 4 close parentheses over denominator 2 up diagonal strike open parentheses x plus 3 close parentheses end strike end fraction

Multiply the remaining numerators and denominators together

fraction numerator 1 cross times open parentheses x plus 4 close parentheses over denominator 1 cross times 2 end fraction equals fraction numerator x plus 4 over denominator 2 end fraction

Check to see if you missed any terms that are the same on the top and bottom that could be cancelled

fraction numerator x plus 4 over denominator 2 end fraction is already in its simplest form

fraction numerator bold italic x bold plus bold 4 over denominator bold 2 end fraction

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Naomi C

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Naomi graduated from Durham University in 2007 with a Masters degree in Civil Engineering. She has taught Mathematics in the UK, Malaysia and Switzerland covering GCSE, IGCSE, A-Level and IB. She particularly enjoys applying Mathematics to real life and endeavours to bring creativity to the content she creates.

Dan Finlay

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Dan graduated from the University of Oxford with a First class degree in mathematics. As well as teaching maths for over 8 years, Dan has marked a range of exams for Edexcel, tutored students and taught A Level Accounting. Dan has a keen interest in statistics and probability and their real-life applications.