OCR C2 2013 January — Question 9 12 marks

Exam BoardOCR
ModuleC2 (Core Mathematics 2)
Year2013
SessionJanuary
Marks12
PaperDownload PDF ↗
Mark schemeDownload PDF ↗
TopicIndefinite & Definite Integrals
TypeIntegration with given constant
DifficultyStandard +0.3 This is a straightforward C2 integration question requiring algebraic simplification before integration, evaluating definite integral, and solving a cubic equation with a given root. All techniques are standard: simplify rational expression, integrate term-by-term, substitute limits, factor cubic using given root, and solve quadratic. Slightly above average difficulty due to multiple steps and algebraic manipulation, but no novel insight required.
Spec1.02j Manipulate polynomials: expanding, factorising, division, factor theorem1.08b Integrate x^n: where n != -1 and sums1.08d Evaluate definite integrals: between limits

9 The positive constant \(a\) is such that \(\int _ { a } ^ { 2 a } \frac { 2 x ^ { 3 } - 5 x ^ { 2 } + 4 } { x ^ { 2 } } \mathrm {~d} x = 0\).
  1. Show that \(3 a ^ { 3 } - 5 a ^ { 2 } + 2 = 0\).
  2. Show that \(a = 1\) is a root of \(3 a ^ { 3 } - 5 a ^ { 2 } + 2 = 0\), and hence find the other possible value of \(a\), giving your answer in simplified surd form.

Question 9(i):
AnswerMarks Guidance
AnswerMarks Guidance
\(\int(2x - 5 + 4x^{-2})\,dx = x^2 - 5x - 4x^{-1}\)M1 Attempt to rewrite integrand in a suitable form. Attempt to divide all 3 terms by \(x^2\), or attempt to multiply all 3 terms by \(x^{-2}\)
Obtain \(2x - 5 + 4x^{-2}\)A1 Allow if third term is written in fractional form
Attempt integration of their integrandM1 Integrand must be written as a polynomial ie with all terms of the form \(kx^n\), and no brackets. At least two terms must increase in power by 1. Allow if the \(-5\) disappears
Obtain \(x^2 - 5x - 4x^{-1}\)A1 Allow unsimplified (eg \(\frac{4}{-1}x^{-1}\))
\((4a^2 - 10a - \frac{2}{a}) - (a^2 - 5a - \frac{4}{a}) = 0\), \(3a^2 - 5a + \frac{2}{a} = 0\), \(3a^3 - 5a^2 + 2 = 0\) AGM1 Attempt use of limits. Must be \(F(2a) - F(a)\) ie subtraction with limits in the correct order. Must be in integration attempt
Equate to 0 and rearrange to obtain \(3a^3 - 5a^2 + 2 = 0\)A1 Must be equated to 0 before multiplying through by \(a\). At least one extra line of working required between \((4a^2-10a-\frac{2}{a})-(a^2-5a-\frac{4}{a})=0\) and final answer. AG so look carefully at working
Question 9(ii):
AnswerMarks Guidance
AnswerMarks Guidance
\(f(1) = 3 - 5 + 2 = 0\) AGB1 Confirm \(f(1)=0\) — detail required. \(3(1)^3 - 5(1)^2 + 2 = 0\) is enough. B0 for just \(f(1)=0\). If using division must show '0' on last line. If using coefficient matching must show '\(R=0\)'. If using inspection there must be some indication of no remainder
\(f(a) = (a-1)(3a^2 - 2a - 2)\)M1 Attempt full division by \((a-1)\), or equiv method. Must be complete method — ie all 3 terms attempted
Obtain \(3a^2\) and one other correct termA1 Could be middle or final term depending on method. Must be correctly obtained
Obtain fully correct quotientA1 For coeff matching it must now be explicit: \(A=3\), \(B=-2\), \(C=-2\)
\(a = \frac{2 \pm \sqrt{4+24}}{6} = \frac{2 \pm 2\sqrt{7}}{6} = \frac{1\pm\sqrt{7}}{3}\)M1 Attempt to solve quadratic. Using quadratic formula or completing the square. M0 if factorising attempt as expected root is a surd. Quadratic must come from division attempt
hence \(a = \frac{1}{3}(1+\sqrt{7})\) onlyA1 Must give the positive root only, so A0 if negative root still present (but condone \(a=1\) also given). Allow aef but must be a simplified surd as per request on question paper (ie simplify \(\sqrt{28}\))
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## Question 9(i):

| Answer | Marks | Guidance |
|--------|-------|----------|
| $\int(2x - 5 + 4x^{-2})\,dx = x^2 - 5x - 4x^{-1}$ | M1 | Attempt to rewrite integrand in a suitable form. Attempt to divide all 3 terms by $x^2$, or attempt to multiply all 3 terms by $x^{-2}$ |
| Obtain $2x - 5 + 4x^{-2}$ | A1 | Allow if third term is written in fractional form |
| Attempt integration of their integrand | M1 | Integrand must be written as a polynomial ie with all terms of the form $kx^n$, and no brackets. At least two terms must increase in power by 1. Allow if the $-5$ disappears |
| Obtain $x^2 - 5x - 4x^{-1}$ | A1 | Allow unsimplified (eg $\frac{4}{-1}x^{-1}$) |
| $(4a^2 - 10a - \frac{2}{a}) - (a^2 - 5a - \frac{4}{a}) = 0$, $3a^2 - 5a + \frac{2}{a} = 0$, $3a^3 - 5a^2 + 2 = 0$ **AG** | M1 | Attempt use of limits. Must be $F(2a) - F(a)$ ie subtraction with limits in the correct order. Must be in integration attempt |
| Equate to 0 and rearrange to obtain $3a^3 - 5a^2 + 2 = 0$ | A1 | Must be equated to 0 before multiplying through by $a$. At least one extra line of working required between $(4a^2-10a-\frac{2}{a})-(a^2-5a-\frac{4}{a})=0$ and final answer. **AG** so look carefully at working |

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## Question 9(ii):

| Answer | Marks | Guidance |
|--------|-------|----------|
| $f(1) = 3 - 5 + 2 = 0$ **AG** | B1 | Confirm $f(1)=0$ — detail required. $3(1)^3 - 5(1)^2 + 2 = 0$ is enough. B0 for just $f(1)=0$. If using division must show '0' on last line. If using coefficient matching must show '$R=0$'. If using inspection there must be some indication of no remainder |
| $f(a) = (a-1)(3a^2 - 2a - 2)$ | M1 | Attempt full division by $(a-1)$, or equiv method. Must be complete method — ie all 3 terms attempted |
| Obtain $3a^2$ and one other correct term | A1 | Could be middle or final term depending on method. Must be correctly obtained |
| Obtain fully correct quotient | A1 | For coeff matching it must now be explicit: $A=3$, $B=-2$, $C=-2$ |
| $a = \frac{2 \pm \sqrt{4+24}}{6} = \frac{2 \pm 2\sqrt{7}}{6} = \frac{1\pm\sqrt{7}}{3}$ | M1 | Attempt to solve quadratic. Using quadratic formula or completing the square. M0 if factorising attempt as expected root is a surd. Quadratic must come from division attempt |
| hence $a = \frac{1}{3}(1+\sqrt{7})$ only | A1 | Must give the positive root only, so A0 if negative root still present (but condone $a=1$ also given). Allow aef but must be a simplified surd as per request on question paper (ie simplify $\sqrt{28}$) |

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9 The positive constant $a$ is such that $\int _ { a } ^ { 2 a } \frac { 2 x ^ { 3 } - 5 x ^ { 2 } + 4 } { x ^ { 2 } } \mathrm {~d} x = 0$.\\
(i) Show that $3 a ^ { 3 } - 5 a ^ { 2 } + 2 = 0$.\\
(ii) Show that $a = 1$ is a root of $3 a ^ { 3 } - 5 a ^ { 2 } + 2 = 0$, and hence find the other possible value of $a$, giving your answer in simplified surd form.

\hfill \mbox{\textit{OCR C2 2013 Q9 [12]}}