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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. The roots of this equation are ω1ω2 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.

Verify that ω1=1+eiπ6 is a root of this equation.

[2]
a.i.

Find ω2 and ω3, expressing these in the form a+eiθ, where a and θ>0.

[4]
a.ii.

Plot the points A, B and C on an Argand diagram.

[4]
b.

Find AC.

[3]
c.

By using de Moivre’s theorem, show that α=11-eiπ6 is a root of this equation.

[3]
d.

Determine the value of Re(α).

[6]
e.

Markscheme

(1+eiπ6-1)3

=(eiπ6)3                   A1

=eiπ2                  A1

=cosπ2+isinπ2

=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]

a.i.

(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]

a.ii.

EITHER

attempt to express eiπ6ei5π6ei9π6 in Cartesian form and translate 1 unit in the positive direction of the real axis                  (M1)


OR

attempt to express w1w2 and w3 in Cartesian form                  (M1)


THEN

Note: To award A marks, it is not necessary to see AB or C, the w1, or the solid lines

                  A1A1A1

 

[4 marks]

b.

valid attempt to find ω1-ω3 (or ω3-ω1)                     M1

ω1-ω3=(1+32+12i)-(1-i)=32+32i  OR  cosπ6+isinπ6+isinπ2

valid attempt to find |32+32i|                     M1

=34+94

AC=3                     A1

 

[3 marks]

c.

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 (=i52-332+i(332-52))                      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]

d.

METHOD 1

11-eiπ6=11-(cosπ6+isinπ6)                    M1

=22-3-i                     A1

attempt to use conjugate to rationalise                    M1

=4-23+2i(2-3)2+1                     A1

=4-23+2i8-43                     A1

=12+14-23i

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+isinπ6)                    M1

attempt to use conjugate to rationalise                    M1

=1(1-cosπ6)-isinπ6×(1-cosπ6)+isinπ6(1-cosπ6)+isinπ6                     A1

=(1-cosπ6)+isinπ6(1-cosπ6)2+sin2π6                     A1

=(1-cosπ6)+isinπ61-2cosπ6+cos2π6+sin2π6

=(1-cosπ6)+isinπ62-2cosπ6                     A1

=12+isinπ62-2cosπ6

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)+isin(-π12)cosπ12+isinπ12-(cos(-π12)+isin(-π12))                     A1

=-cosπ12-isinπ122isinπ12                     A1

=12-12icotπ12 (=12+12icotπ12)

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-isinπ62-2cosπ6                    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]

e.

Examiners report

[N/A]
a.i.
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a.ii.
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b.
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c.
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d.
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e.

Syllabus sections

Topic 1—Number and algebra » AHL 1.12—Complex numbers – Cartesian form and Argand diag
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