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A-LevelMathematicsComplex numbers9709/32 · Oct/Nov 2018 · Q910 Marks

Without using a calculator, express the complex number (2 + 6i)/(1-2i) in the form x + iy, where x and y are real.

  1. (a)(i)Without using a calculator, express the complex number (2 + 6i)/(1-2i) in the form x + iy, where x and y are real.[2]
  2. (a)(ii)Hence, without using a calculator, express (2 + 6i)/(1-2i) in the form r(cos θ + i sin θ), where r > 0 and -π < θ ≤ π, giving the exact values of r and θ.[3]
  3. (b)On a sketch of an Argand diagram, shade the region whose points represent complex numbers z satisfying both the inequalities |z – 3i| ≤ 1 and Re z ≤ 0, where Re z denotes the real part of z. Find the greatest value of arg z for points in this region, giving your answer in radians correct to 2 decimal places.[5]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2018). The mark or mark type is shown in brackets.

  1. [M1] Multiply numerator and denominator by 1 + 2i, or equivalent
  2. [A1] Obtain quotient – 2 + 2i
  3. [M1] Use correct method to find either r or θ
  4. [A1ft] Obtain r = 2√2, or exact equivalent
  5. [A1ft] Obtain θ = 3π/4 from exact work
  6. [B1] Show a circle with centre 3i
  7. [B1ft] Show a circle with radius 1
  8. [B1] All correct with even scales and shade the correct region
  9. [M1] Carry out a correct method for calculating greatest value of arg z
  10. [A1] Obtain answer 1.91

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About this question

Topic
Complex numbers, A-Level Mathematics (syllabus 9709). Structured question, 10 marks.
Source
A-Level Mathematics, Oct/Nov 2018 Paper 3 Variant 2, question 9.

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