Linear Transformations of Roots (Edexcel A Level Further Maths: Core Pure)

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Jamie W

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Jamie W

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Linear Transformations of Roots

What is a Linear Transformation?

  • A linear transformation can be expressed as w equals p x plus q where:
    • w is the new root
    • x is the original root
    • p and q are constants
  • If you perform a linear transformation on a polynomial equation, the transformed equation’s roots will be linked directly to the original roots
  • You can think of this as a translation and/or a stretch of the polynomial and its roots

How do I find the new transformed polynomial?

  • STEP 1
    Rewrite the transformation w equals p x plus q as x equals fraction numerator w minus q over denominator p end fraction
  • STEP 2
    Make the substitution x equals fraction numerator w minus q over denominator p end fraction into the original polynomial
  • STEP 3
    Expand and multiply by a constant to make all the coefficients integers (if necessary or desirable) and swap the w back for an x in your final answer
  • Remember that your solution is not unique.  Multiplying the entire polynomial by a constant will produce a different polynomial, but will not affect the roots

Examiner Tip

  • Check the question to see if you are required to expand and simplify your final answer or not, as it can be time consuming!
  • If you are required to expand and simplify, make use of the binomial expansion to make the process much quicker
  • Use your calculator’s polynomial solver to check the solutions of the original equation and your new equation, to make sure they are related to each other as described

Worked example

The cubic equation x cubed minus 7 x squared plus 2 x plus 40 equals 0 has roots alpha comma beta comma gamma. Find a polynomial equation with roots:

a)
3 alpha comma space 3 beta comma space 3 gamma

3-1-2-edx-a-fm-we1a-soltn

b)
left parenthesis 2 alpha minus 1 right parenthesis comma space left parenthesis 2 beta minus 1 right parenthesis comma space left parenthesis 2 gamma minus 1 right parenthesis

3-1-2-edx-a-fm-we1b-soltn

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Jamie W

Author: Jamie W

Expertise: Maths

Jamie graduated in 2014 from the University of Bristol with a degree in Electronic and Communications Engineering. He has worked as a teacher for 8 years, in secondary schools and in further education; teaching GCSE and A Level. He is passionate about helping students fulfil their potential through easy-to-use resources and high-quality questions and solutions.