Loading [MathJax]/jax/output/CommonHTML/fonts/TeX/fontdata.js

User interface language: English | Español

Date May 2012 Marks available 7 Reference code 12M.1.hl.TZ1.3
Level HL only Paper 1 Time zone TZ1
Command term Find, Hence, and Express Question number 3 Adapted from N/A

Question

If z1=a+a3i and z2=1i, where a is a real constant, express z1 and z2 in the form rcisθ, and hence find an expression for (z1z2)6 in terms of a and i.

Markscheme

z1=2acis(π3)z2=2 cis(π4)     M1     A1     A1

EITHER

(z1z2)6=26a6cis(0)26cis(π2)(=8a6cis(π2))     M1     A1     A1

OR

(z1z2)6=(2a2cis(7π12))6     M1     A1

=8a6cis(π2)     A1

THEN

=8a6i     A1

Note: Accept equivalent angles, in radians or degrees. 

Accept alternate answers without cis e.g.  = 8a6i

[7 marks]

Examiners report

Most students had an idea of what to do but were frequently let down in their calculations of the modulus and argument. The most common error was to give the argument of z2 as 3π4, failing to recognise that it should be in the fourth quadrant. There were also errors seen in the algebraic manipulation, in particular forgetting to raise the modulus to the power 6.

Syllabus sections

Topic 1 - Core: Algebra » 1.5 » Complex numbers: the number i=1 ; the terms real part, imaginary part, conjugate, modulus and argument.
Show 33 related questions

View options