Date | May 2019 | Marks available | 1 | Reference code | 19M.2.AHL.TZ1.H_6 |
Level | Additional Higher Level | Paper | Paper 2 | Time zone | Time zone 1 |
Command term | Find | Question number | H_6 | Adapted from | N/A |
Question
Let z=a+bi, a, b∈R+ and let argz=θ.
Show the points represented by z and z−2a on the following Argand diagram.
Find an expression in terms of θ for arg(z−2a).
Find an expression in terms of θ for arg(zz−2a).
Hence or otherwise find the value of θ for which Re(zz−2a)=0.
Markscheme
* This question is from an exam for a previous syllabus, and may contain minor differences in marking or structure.
A1
Note: Award A1 for z in first quadrant and z−2a its reflection in the y-axis.
[1 mark]
π−θ (or any equivalent) A1
[1 mark]
arg(zz−2a)=arg(z)−arg(z−2a) (M1)
=2θ−π (or any equivalent) A1
[2 marks]
METHOD 1
if Re(zz−2a)=0 then 2θ−π=nπ2, (n odd) (M1)
−π<2θ−π<0⇒n=−1
2θ−π=−π2 (A1)
θ=π4 A1
METHOD 2
a+bi−a+bi=b2−a2−2abia2+b2 M1
Re(zz−2a)=0⇒b2−a2=0
b=a A1
θ=π4 A1
Note: Accept any equivalent, eg θ=−7π4.
[3 marks]