Edexcel CP2 2023 June — Question 8

Exam BoardEdexcel
ModuleCP2 (Core Pure 2)
Year2023
SessionJune
TopicComplex Numbers Argand & Loci

  1. Given that a cubic equation has three distinct roots that all lie on the same straight line in the complex plane,
    1. describe the possible lines the roots can lie on.
    $$f ( z ) = 8 z ^ { 3 } + b z ^ { 2 } + c z + d$$ where \(b , c\) and \(d\) are real constants.
    The roots of \(f ( z )\) are distinct and lie on a straight line in the complex plane.
    Given that one of the roots is \(\frac { 3 } { 2 } + \frac { 3 } { 2 } \mathrm { i }\)
  2. state the other two roots of \(\mathrm { f } ( \mathrm { z } )\) $$g ( z ) = z ^ { 3 } + P z ^ { 2 } + Q z + 12$$ where \(P\) and \(Q\) are real constants, has 3 distinct roots.
    The roots of \(g ( z )\) lie on a different straight line in the complex plane than the roots of \(\mathrm { f } ( \mathrm { z } )\) Given that
    • \(f ( z )\) and \(g ( z )\) have one root in common
    • one of the roots of \(\mathrm { g } ( \mathrm { z } )\) is - 4
      1. write down the value of the common root,
      2. determine the value of the other root of \(\mathrm { g } ( \mathrm { z } )\)
    • Hence solve the equation \(\mathrm { f } ( \mathrm { z } ) = \mathrm { g } ( \mathrm { z } )\)