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Date May Specimen paper Marks available 5 Reference code SPM.1.AHL.TZ0.11
Level Additional Higher Level Paper Paper 1 (without calculator) Time zone Time zone 0
Command term Express Question number 11 Adapted from N/A

Question

Let the roots of the equation  z 3 = 3 + 3 i be  u v and  w .

On an Argand diagram, u v and  w  are represented by the points U, V and W respectively.

Express  3 + 3 i in the form  r e i θ , where  r > 0 and π < θ π .

[5]
a.

Find  u v and  w  expressing your answers in the form  r e i θ , where  r > 0 and  π < θ π .

[5]
b.

Find the area of triangle UVW.

[4]
c.

By considering the sum of the roots u v and  w , show that

cos 5 π 18 + cos 7 π 18 + cos 17 π 18 = 0 .

[4]
d.

Markscheme

attempt to find modulus      (M1)

r = 2 3 ( = 12 )       A1

attempt to find argument in the correct quadrant      (M1)

θ = π + arctan ( 3 3 )       A1

= 5 π 6       A1

3 + 3 i = 12 e 5 π i 6 ( = 2 3 e 5 π i 6 )

[5 marks]

a.

attempt to find a root using de Moivre’s theorem      M1

12 1 6 e 5 π i 18        A1

attempt to find further two roots by adding and subtracting  2 π 3 to the argument    M1

12 1 6 e 7 π i 18        A1

12 1 6 e 17 π i 18        A1

Note: Ignore labels for u v and  w at this stage.

 

[5 marks]

b.

METHOD 1
attempting to find the total area of (congruent) triangles UOV, VOW and UOW        M1

Area = 3 ( 1 2 ) ( 12 1 6 ) ( 12 1 6 ) sin 2 π 3       A1A1

Note: Award A1 for  ( 12 1 6 ) ( 12 1 6 )  and A1 for  sin 2 π 3

= 3 3 4 ( 12 1 3 )  (or equivalent)     A1

 

METHOD 2

UV2  = ( 12 1 6 ) 2 + ( 12 1 6 ) 2 2 ( 12 1 6 ) ( 12 1 6 ) cos 2 π 3  (or equivalent)     A1

UV  = 3 ( 12 1 6 )  (or equivalent)     A1

attempting to find the area of UVW using Area =  1 2  × UV × VW × sin  α  for example        M1

Area  = 1 2 ( 3 × 12 1 6 ) ( 3 × 12 1 6 ) sin π 3

= 3 3 4 ( 12 1 3 )  (or equivalent)     A1

 

[4 marks]

c.

u + v + w = 0     R1

12 1 6 ( cos ( 7 π 18 ) + i sin ( 7 π 18 ) + cos 5 π 18 + i sin 5 π 18 + cos 17 π 18 + i sin 17 π 18 ) = 0      A1

consideration of real parts       M1

12 1 6 ( cos ( 7 π 18 ) + cos 5 π 18 + cos 17 π 18 ) = 0

cos ( 7 π 18 ) = cos 17 π 18  explicitly stated      A1

cos 5 π 18 + cos 7 π 18 + cos 17 π 18 = 0      AG

[4 marks]

d.

Examiners report

[N/A]
a.
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b.
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c.
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d.

Syllabus sections

Topic 2—Functions » AHL 2.12—Factor and remainder theorems, sum and product of roots
Show 28 related questions
Topic 1—Number and algebra » AHL 1.13—Polar and Euler form
Topic 1—Number and algebra
Topic 2—Functions

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