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, w∈C. Give your answers in Cartesian form.
One of the roots w1 satisfies the condition Re(w1)=0.
Given that w1=zz−i, express z in the form a+bi, where a, b∈Q.
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)(w2−2wi−4)=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]
w1=−2i
zz−i=−2i M1
z=−2i(z−i)
z(1+2i)=−2
z=−21+2i A1
z=−25+45i A1
Note: Accept a=−25,b=45.
[3 marks]