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+a√3i and z2=1−i, 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=(2a√2cis(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.