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 sinπ2
=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]
(z-1)3=ei(π2+2πk) (M1)
z-1=ei(π6+4πk6) (M1)
(k=1)⇒ω2=1+ei5π6 A1
(k=2)⇒ω3=1+ei9π6 A1
[4 marks]
EITHER
attempt to express eiπ6, ei5π6, ei9π6 in Cartesian form and translate 1 unit in the positive direction of the real axis (M1)
OR
attempt to express w1, w2 and w3 in Cartesian form (M1)
THEN
Note: To award A marks, it is not necessary to see A, B or C, the w1, or the solid lines
A1A1A1
[4 marks]
valid attempt to find ω1-ω3 (or ω3-ω1) M1
ω1-ω3=(1+√32+12i)-(1-i)=√32+32i OR cosπ6+i sinπ6+i sinπ2
valid attempt to find |√32+32i| M1
=√34+94
AC=√3 A1
[3 marks]
METHOD 1
(z-1)3=iz3⇒(z-1z)3=i M1
(z-1z)3=eiπ2 A1
α-1α=eiπ6 A1
Note: This step to change from z to α may occur at any point in MS.
α-1=αeiπ6
α-αeiπ6=1
α(1-eiπ6)=1
α=11-eiπ6 AG
METHOD 2
(z-1)3=iz3⇒(z-1z)3=i M1
(1-1z)3=eiπ2 A1
1-1z=eiπ6 A1
Note: This step to change from z to α may occur at any point in MS.
1-eiπ6=1α
α=11-eiπ6 AG
METHOD 3
LHS=(z-1)3=(11-eiπ6-1)3
=(eiπ61-eiπ6)3
=i(1-eiπ6)3 (=i52-3√32+i(3√32-52)) M1A1
Note: Award M1 for applying de Moivre’s theorem (may be seen in modulus- argument form.)
RHS=iz3=i(11-eiπ6)3
=i(1-eiπ6)3 A1
(z-1)3=iz3 AG
METHOD 4
(z-1)3=iz3
z3-3z2+3z-1=iz3
(1-i)z3-3z2+3z-1=0 (M1)
(1-i)(11-eiπ6)3-3(11-eiπ6)2+3(11-eiπ6)-1
=(1-i)-3(1-eiπ6)+3(1-eiπ6)2-(1-eiπ6)3 (A1)
=(1-i)-3(1-eiπ6)+3(1-2eiπ6+eiπ3)-(1-3eiπ6+3eiπ3-eiπ2) A1
=0 AG
Note: If the candidate does not interpret their conclusion, award (M1)(A1)A0 as appropriate.
[3 marks]
METHOD 1
11-eiπ6=11-(cosπ6+i sinπ6) M1
=22-√3-i A1
attempt to use conjugate to rationalise M1
=4-2√3+2i(2-√3)2+1 A1
=4-2√3+2i8-4√3 A1
=12+14-2√3i
⇒Re(α)=12 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
11-eiπ6=11-(cosπ6+i sinπ6) M1
attempt to use conjugate to rationalise M1
=1(1-cosπ6)-i sinπ6×(1-cosπ6)+i sinπ6(1-cosπ6)+i sinπ6 A1
=(1-cosπ6)+i sinπ6(1-cosπ6)2+sin2π6 A1
=(1-cosπ6)+i sinπ61-2 cosπ6+cos2π6+sin2π6
=(1-cosπ6)+i sinπ62-2 cosπ6 A1
=12+i sinπ62-2 cosπ6
⇒Re(α)=12 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 -e-iπ12e-iπ12 M1
11-eiπ6=-e-iπ12eiπ12-e-iπ12 A1
attempting to re-write in r-cis form M1
=-cos(-π12)+i sin(-π12)cosπ12+i sinπ12-(cos(-π12)+i sin(-π12)) A1
=-cosπ12-i sinπ122i sinπ12 A1
=12-12icotπ12 (=12+12i cotπ12)
⇒Re(α)=12 A1
METHOD 4
attempt to multiply through by 1-e-iπ61-e-iπ6 M1
11-eiπ6=1-e-iπ61-e-iπ6-eiπ6+1 A1
attempting to re-write in r-cis form M1
=1-cosπ6-i sinπ62-2 cosπ6 A1
attempt to re-write in Cartesian form M1
=1-√32-12i2-√3 (=2-√322-√3+i122-√3)
⇒Re(α)=12 A1
Note: Their final imaginary part does not have to be correct in order for the final A mark to be awarded
[6 marks]