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 u, v and w.
On an Argand diagram, u, v and w are represented by the points U, V and W respectively.
Express −3+√3i in the form reiθ, where r>0 and −π<θ⩽π.
Find u, v and w expressing your answers in the form reiθ, where r>0 and −π<θ⩽π.
Find the area of triangle UVW.
By considering the sum of the roots u, v and w, show that
cos5π18+cos7π18+cos17π18=0.
Markscheme
attempt to find modulus (M1)
r=2√3(=√12) A1
attempt to find argument in the correct quadrant (M1)
θ=π+arctan(−√33) A1
=5π6 A1
−3+√3i=√12e5πi6(=2√3e5πi6)
[5 marks]
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
1216e−7πi18 A1
1216e17πi18 A1
Note: Ignore labels for u, v and w at this stage.
[5 marks]
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
= 3√34(1213) (or equivalent) A1
METHOD 2
UV2 =(1216)2+(1216)2−2(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
= 3√34(1213) (or equivalent) A1
[4 marks]
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]