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A-LevelMathematicsComplex numbers9709/31 · May/June 2019 · Q1013 Marks

Throughout this question the use of a calculator is not permitted. The complex number (√3) + i is denoted by u.

  1. (i)Express u in the form r e^(iθ), where r > 0 and −π < θ ≤ π, giving the exact values of r and θ. Hence or otherwise state the exact values of the modulus and argument of u⁴.[5]
  2. (ii)Verify that u is a root of the equation z³ – 8z + 8√3 = 0 and state the other complex root of this equation.[3]
  3. (iii)On a sketch of an Argand diagram, shade the region whose points represent complex numbers z satisfying the inequalities |z - u| ≤ 2 and Im z ≥ 2, where Im z denotes the imaginary part of z.[5]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

  1. [B1] (i) State or imply r = 2
  2. [B1] (i) State or imply θ = ⅙π
  3. [M1] (i) Use a correct method for finding the modulus or the argument of u⁴
  4. [A1] (i) Obtain modulus 16
  5. [A1] (i) Obtain argument ⅔π
  6. [M1] (ii) Substitute u and carry out a correct method for finding u³
  7. [A1] (ii) Verify u is a root of the given equation
  8. [B1] (ii) State that the other root is √3-i
  9. [B1] (iii) Show the point representing u in a relatively correct position
  10. [B1] (iii) Show a circle with centre u and radius 2
  11. [B1] (iii) Show the line y = 2
  12. [B1] (iii) Shade the correct region
  13. [B1] (iii) Show that the line and circle intersect on x = 0

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About this question

Topic
Complex numbers, A-Level Mathematics (syllabus 9709). Structured question, 13 marks.
Source
A-Level Mathematics, May/June 2019 Paper 3 Variant 1, question 10.

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