Date | November 2021 | Marks available | 3 | Reference code | 21N.1.AHL.TZ0.12 |
Level | Additional Higher Level | Paper | Paper 1 (without calculator) | Time zone | Time zone 0 |
Command term | Show that | Question number | 12 | Adapted from | N/A |
Question
Consider the equation (z-1)3=i, z∈ℂ. The roots of this equation are ω1, ω2 and ω3, where Im(ω2)>0 and Im(ω3)<0.
The roots ω1, ω2 and ω3 are represented by the points A, B and C respectively on an Argand diagram.
Consider the equation (z-1)3=iz3, z∈ℂ.
Verify that ω1=1+eiπ6 is a root of this equation.
Find ω2 and ω3, expressing these in the form a+eiθ, where a∈ℝ and θ>0.
Plot the points A, B and C on an Argand diagram.
Find AC.
By using de Moivre’s theorem, show that α=11-eiπ6 is a root of this equation.
Determine the value of Re(α).
Markscheme
(1+eiπ6-1)3
=(eiπ6)3 A1
=eiπ2 A1
=cosπ2+i
AG
Note: Candidates who solve the equation correctly can be awarded the above two marks. The working for part (i) may be seen in part (ii).
[2 marks]
(M1)
(M1)
A1
A1
[4 marks]
EITHER
attempt to express , , in Cartesian form and translate 1 unit in the positive direction of the real axis (M1)
OR
attempt to express , and in Cartesian form (M1)
THEN
Note: To award A marks, it is not necessary to see , or , the , or the solid lines
A1A1A1
[4 marks]
valid attempt to find M1
OR
valid attempt to find M1
A1
[3 marks]
METHOD 1
M1
A1
A1
Note: This step to change from to may occur at any point in MS.
AG
METHOD 2
M1
A1
A1
Note: This step to change from to may occur at any point in MS.
AG
METHOD 3
LHS
M1A1
Note: Award M1 for applying de Moivre’s theorem (may be seen in modulus- argument form.)
RHS
A1
AG
METHOD 4
(M1)
(A1)
A1
AG
Note: If the candidate does not interpret their conclusion, award (M1)(A1)A0 as appropriate.
[3 marks]
METHOD 1
M1
A1
attempt to use conjugate to rationalise M1
A1
A1
A1
Note: Their final imaginary part does not have to be correct in order for the final three A marks to be awarded
METHOD 2
M1
attempt to use conjugate to rationalise M1
A1
A1
A1
A1
Note: Their final imaginary part does not have to be correct in order for the final three A marks to be awarded
METHOD 3
attempt to multiply through by M1
A1
attempting to re-write in r-cis form M1
A1
A1
A1
METHOD 4
attempt to multiply through by M1
A1
attempting to re-write in r-cis form M1
A1
attempt to re-write in Cartesian form M1
A1
Note: Their final imaginary part does not have to be correct in order for the final A mark to be awarded
[6 marks]