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Date May Specimen paper Marks available 4 Reference code SPM.1.AHL.TZ0.11
Level Additional Higher Level Paper Paper 1 (without calculator) Time zone Time zone 0
Command term Show that Question number 11 Adapted from N/A

Question

Let the roots of the equation z3=3+3i be uv and w.

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

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

[5]
a.

Find uv and w expressing your answers in the form reiθ, where r>0 and π<θπ.

[5]
b.

Find the area of triangle UVW.

[4]
c.

By considering the sum of the roots uv and w, show that

cos5π18+cos7π18+cos17π18=0.

[4]
d.

Markscheme

attempt to find modulus      (M1)

r=23(=12)      A1

attempt to find argument in the correct quadrant      (M1)

θ=π+arctan(33)      A1

=5π6      A1

3+3i=12e5πi6(=23e5πi6)

[5 marks]

a.

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

1216e5πi18       A1

attempt to find further two roots by adding and subtracting 2π3 to the argument    M1

1216e7πi18       A1

1216e17πi18       A1

Note: Ignore labels for uv 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(12)(1216)(1216)sin2π3      A1A1

Note: Award A1 for (1216)(1216) and A1 for sin2π3

= 334(1213) (or equivalent)     A1

 

METHOD 2

UV2 =(1216)2+(1216)22(1216)(1216)cos2π3 (or equivalent)     A1

UV =3(1216) (or equivalent)     A1

attempting to find the area of UVW using Area = 12 × UV × VW × sin α for example        M1

Area =12(3×1216)(3×1216)sinπ3

= 334(1213) (or equivalent)     A1

 

[4 marks]

c.

u + v + w = 0     R1

1216(cos(7π18)+isin(7π18)+cos5π18+isin5π18+cos17π18+isin17π18)=0     A1

consideration of real parts       M1

1216(cos(7π18)+cos5π18+cos17π18)=0

cos(7π18)=cos17π18 explicitly stated      A1

cos5π18+cos7π18+cos17π18=0     AG

[4 marks]

d.

Examiners report

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