CAIE P3 2022 March — Question 6 6 marks

Exam BoardCAIE
ModuleP3 (Pure Mathematics 3)
Year2022
SessionMarch
Marks6
PaperDownload PDF ↗
Mark schemeDownload PDF ↗
TopicComplex Numbers Arithmetic
TypeQuadratic equations involving z² and z*
DifficultyChallenging +1.2 This question requires substituting w = x + iy and w* = x - iy into the equation, then separating real and imaginary parts to form simultaneous equations. While it involves multiple algebraic steps and careful manipulation of complex conjugates, it follows a standard technique taught in P3. The constraint Re(w) ≤ 0 adds minimal difficulty as it simply filters solutions. More challenging than routine complex arithmetic but less demanding than problems requiring geometric insight or proof.
Spec4.02i Quadratic equations: with complex roots

6 Find the complex numbers \(w\) which satisfy the equation \(w ^ { 2 } + 2 \mathrm { i } w ^ { * } = 1\) and are such that \(\operatorname { Re } w \leqslant 0\). Give your answers in the form \(x + \mathrm { i } y\), where \(x\) and \(y\) are real.

Question 6:
AnswerMarks Guidance
AnswerMark Guidance
Substitute and obtain a correct equation in \(x\) and \(y\)B1 \((x+iy)^2 + 2i(x - iy) = 1\)
Use \(i^2 = -1\) at least once and equate real and imaginary partsM1
Obtain two correct equations, e.g. \(x^2 - y^2 + 2y = 1\) and \(2xy + 2x = 0\)A1
Solve for \(x\) or for \(y\)M1
Using \(y = -1\), obtain answer \(w = -2 - i\) onlyA1 A0 if \(w = 2 - i\) as well
Using \(x = 0\), obtain answer \(w = i\)A1
Total: 6
## Question 6:

| Answer | Mark | Guidance |
|--------|------|----------|
| Substitute and obtain a correct equation in $x$ and $y$ | B1 | $(x+iy)^2 + 2i(x - iy) = 1$ |
| Use $i^2 = -1$ at least once and equate real and imaginary parts | M1 | |
| Obtain two correct equations, e.g. $x^2 - y^2 + 2y = 1$ and $2xy + 2x = 0$ | A1 | |
| Solve for $x$ or for $y$ | M1 | |
| Using $y = -1$, obtain answer $w = -2 - i$ only | A1 | A0 if $w = 2 - i$ as well |
| Using $x = 0$, obtain answer $w = i$ | A1 | |
| **Total: 6** | | |
6 Find the complex numbers $w$ which satisfy the equation $w ^ { 2 } + 2 \mathrm { i } w ^ { * } = 1$ and are such that $\operatorname { Re } w \leqslant 0$. Give your answers in the form $x + \mathrm { i } y$, where $x$ and $y$ are real.\\

\hfill \mbox{\textit{CAIE P3 2022 Q6 [6]}}