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Date May 2022 Marks available 3 Reference code 22M.1.AHL.TZ1.9
Level Additional Higher Level Paper Paper 1 (without calculator) Time zone Time zone 1
Command term Find Question number 9 Adapted from N/A

Question

Consider the complex numbers z1=1+bi and z2=1-b2-2bi, where b, b0.

Find an expression for z1z2 in terms of b.

[3]
a.

Hence, given that argz1z2=π4, find the value of b.

[3]
b.

Markscheme

z1z2=1+bi1-b2-2bi

=1-b2-2i2b2+i-2b+b-b3             M1

=1+b2+i-b-b3            A1A1


Note: Award A1 for 1+b2 and A1 for -bi-b3i.

 

[3 marks]

a.

argz1z2=arctan-b-b31+b2=π4            (M1)


EITHER
arctan-b=π4 (since 1+b20, for b)            A1


OR

-b-b3=1+b2  (or equivalent)            A1


THEN

b=-1            A1

 

[3 marks]

b.

Examiners report

Part (a) was generally well done with many completely correct answers seen. Part (b) proved to be challenging with many candidates incorrectly equating the ratio of their imaginary and real parts to π4 instead of tanπ4. Stronger candidates realized that when θ=π4, it forms an isosceles right-angled triangle and equated the real and imaginary parts to obtain the value of b .

a.
[N/A]
b.

Syllabus sections

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

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