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Date May 2019 Marks available 4 Reference code 19M.2.AHL.TZ2.H_8
Level Additional Higher Level Paper Paper 2 Time zone Time zone 2
Command term Find Question number H_8 Adapted from N/A

Question

Find the roots of the equation w3=8i, wC. Give your answers in Cartesian form.

[4]
a.

One of the roots w1 satisfies the condition Re(w1)=0.

Given that w1=zzi, express z in the form a+bi, where a, bQ.

[3]
b.

Markscheme

* This question is from an exam for a previous syllabus, and may contain minor differences in marking or structure.

METHOD 1

w3=8i

writing 8i=8(cos(π2+2πk)+isin(π2+2πk))              (M1)

Note: Award M1 for an attempt to find cube roots of w using modulus-argument form.

cube roots  w=2(cos(π2+2πk3)+isin(π2+2πk3))              (M1)

i.e. w=3+i,3+i,2i         A2

Note: Award A2 for all 3 correct, A1 for 2 correct.

Note: Accept w=1.73+i and w=1.73+i.

 

METHOD 2

w3+(2i)3=0

(w+2i)(w22wi4)=0              M1

w=2i±122              M1

w=3+i,3+i,2i         A2

Note: Award A2 for all 3 correct, A1 for 2 correct.

Note: Accept w=1.73+i and w=1.73+i.

 

[4 marks]

a.

w1=2i

zzi=2i      M1

z=2i(zi)

z(1+2i)=2

z=21+2i      A1

z=25+45i      A1

Note: Accept a=25,b=45.

[3 marks]

b.

Examiners report

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

Syllabus sections

Topic 1—Number and algebra » AHL 1.13—Polar and Euler form
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Topic 1—Number and algebra » AHL 1.14—Complex roots of polynomials, conjugate roots, De Moivre’s, powers & roots of complex numbers
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