OCR FP1 2012 June — Question 3 4 marks

Exam BoardOCR
ModuleFP1 (Further Pure Mathematics 1)
Year2012
SessionJune
Marks4
PaperDownload PDF ↗
Mark schemeDownload PDF ↗
TopicComplex Numbers Arithmetic
TypeQuadratic from one complex root
DifficultyEasy -1.2 This is a straightforward application of the conjugate root theorem for quadratics with real coefficients. Students need only recall that complex roots come in conjugate pairs, then use sum and product of roots to find a and b. It's a standard textbook exercise requiring minimal problem-solving, though the Further Maths context places it slightly above the easiest recall questions.
Spec4.02g Conjugate pairs: real coefficient polynomials

3 One root of the quadratic equation \(x ^ { 2 } + a x + b = 0\), where \(a\) and \(b\) are real, is the complex number \(4 - 3 \mathrm { i }\). Find the values of \(a\) and \(b\).

Question 3:
EITHER
AnswerMarks Guidance
M1Use sum of root and conjugate
\(a = -8\)A1 Obtain correct answer
M1Use product of root and conjugate
\(b = 25\)A1 Obtain correct answer
[4]
OR
AnswerMarks Guidance
M1Substitute \(4 + 3i\) or conjugate into equation
M1Equate real and imaginary parts
\(a = -8\)A1 Obtain correct answer
\(b = 25\)A1 Obtain correct answer
## Question 3:

**EITHER**
| M1 | Use sum of root and conjugate
$a = -8$ | A1 | Obtain correct answer
| M1 | Use product of root and conjugate
$b = 25$ | A1 | Obtain correct answer
**[4]**

**OR**
| M1 | Substitute $4 + 3i$ or conjugate into equation
| M1 | Equate real and imaginary parts
$a = -8$ | A1 | Obtain correct answer
$b = 25$ | A1 | Obtain correct answer

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3 One root of the quadratic equation $x ^ { 2 } + a x + b = 0$, where $a$ and $b$ are real, is the complex number $4 - 3 \mathrm { i }$. Find the values of $a$ and $b$.

\hfill \mbox{\textit{OCR FP1 2012 Q3 [4]}}