Without using a calculator, express the complex number (2 + 6i)/(1-2i) in the form x + iy, where x and y are real.
- (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]
- (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]
- (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.
- [M1] Multiply numerator and denominator by 1 + 2i, or equivalent
- [A1] Obtain quotient – 2 + 2i
- [M1] Use correct method to find either r or θ
- [A1ft] Obtain r = 2√2, or exact equivalent
- [A1ft] Obtain θ = 3π/4 from exact work
- [B1] Show a circle with centre 3i
- [B1ft] Show a circle with radius 1
- [B1] All correct with even scales and shade the correct region
- [M1] Carry out a correct method for calculating greatest value of arg z
- [A1] Obtain answer 1.91
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