OCR C2 2011 June — Question 8 11 marks

Exam BoardOCR
ModuleC2 (Core Mathematics 2)
Year2011
SessionJune
Marks11
PaperDownload PDF ↗
Mark schemeDownload PDF ↗
TopicFunction Transformations
TypeIdentify transformation from equations
DifficultyModerate -0.8 This is a straightforward multi-part question testing basic function transformations, y-intercepts, solving exponential equations with logarithms, and trapezium rule application. All parts are routine C2 techniques requiring direct application of standard methods with no problem-solving insight needed. Slightly easier than average due to the step-by-step scaffolding and standard textbook-style subquestions.
Spec1.02w Graph transformations: simple transformations of f(x)1.06a Exponential function: a^x and e^x graphs and properties1.06g Equations with exponentials: solve a^x = b1.09f Trapezium rule: numerical integration

8 \includegraphics[max width=\textwidth, alt={}, center]{4d03f4e3-ae6c-4a0d-ae4d-d89258d2919a-4_417_931_255_607} The diagram shows the curve \(y = 2 ^ { x } - 3\).
  1. Describe the geometrical transformation that transforms the curve \(y = 2 ^ { x }\) to the curve \(y = 2 ^ { x } - 3\).
  2. State the \(y\)-coordinate of the point where the curve \(y = 2 ^ { x } - 3\) crosses the \(y\)-axis.
  3. Find the \(x\)-coordinate of the point where the curve \(y = 2 ^ { x } - 3\) crosses the \(x\)-axis, giving your answer in the form \(\log _ { a } b\).
  4. The curve \(y = 2 ^ { x } - 3\) passes through the point ( \(p , 62\) ). Use logarithms to find the value of \(p\), correct to 3 significant figures.
  5. Use the trapezium rule, with 2 strips each of width 0.5 , to find an estimate for \(\int _ { 3 } ^ { 4 } \left( 2 ^ { x } - 3 \right) \mathrm { d } x\). Give your answer correct to 3 significant figures.

AnswerMarks Guidance
(i) translation of 3 units in negative y-directionB1 State translation.
State or imply 3 units in negative y-directionB1 2
Give B0D on double negatives eg 'down the y-axis by -3 units' unless clearly wrong or contradictory eg 'negative y-direction by \(\begin{pmatrix} -3 \\ 0 \end{pmatrix}\)'.
AnswerMarks Guidance
(ii) \(y = -2\)B1 1
(-2, 0) is B0 unless -2 already seen or implied as y-coordinate.
AnswerMarks Guidance
(iii) \(2^x = 3\), \(x = \log_2 3\)M1 Attempt to solve \(2^x - 3 = 0\). Rearrange to \(2^x = 3\), introduce logarithms (could be no base or any base as long as consistent) and then attempt expression for \(x\).
M0 for \(x = \log_2 2\). M1 A0 for alternative, correct, log expressions such as \(\log_3 2\) or \(1/\log_3 2\).
Decimal equivalent of 1.58 can get M1 A0.
\(x = \log_3 (y + 3)\) is M0 (unless \(y\) then becomes 0).
AnswerMarks Guidance
State \(\log_2 3\)A1 2
NB \(x - \log_2 3 = 0\) leading to correct answer, can get full marks as there is no incorrect statement seen.
AnswerMarks Guidance
(iv) \(2^p = 65\), \(\log 2^p = \log 65\), \(p\log 2 = \log 65\), \(p = 6.02\)M1* Rearrange equation and introduce logs (or \(\log_2\)). Must first rearrange to \(2^p = k\), with \(k\) from attempt at \(62 \pm 3\), before introducing logs.
Drop power and attempt to solveM1d* Dependent on first M1. If taking logs to any other base, or no base, or \(\log_2\) on both sides then need to drop power of \(p\) and attempt to solve using a sound algebraic method ie \(p = \frac{\log k}{\log 2}\).
Obtain 6.02, or betterA1 3
(v) \(0.5 \times 0.5 \left[ 2^3 - 3 + 2(2^{3.5} - 3) + 2^4 - 3 \right] = 8.66\)M1 Attempt y-values at \(x = 3, 3.5, 4\). M0 if other y-values also found (unless not used in trap rule). Allow M1 for using incorrect function as long as still clearly y-values that are intended to be the original function eg \(2x - 3\) or \(2^{(y-3)}\).
Attempt correct trapezium ruleM1 Must be correct structure ie \(0.5 \times 0.5 \times (y_0 + 2y_1 + y_2)\). Must be finding area from 3 to 4, so using eg \(x = 0, 0.5, 1\) is M0. Allow if still in terms of \(y_i\) etc as long as these have been clearly defined elsewhere. Using x-values in trapezium rule is M0, even if labelled y-values. Allow a different number of strips (except 1) as long as their \(h\) is consistent with this, and the limits are still 3 and 4.
Obtain 8.66, or betterA1 3
Exact answer from integrating \(e^{\ln 2 - 3}\) is 0/3.
Answer only is 0/3.
Attempting integration before using trapezium rule is 0/3.
Using two separate trapezia is fine.
**(i)** translation of 3 units in negative y-direction | B1 | State translation.

**State or imply 3 units in negative y-direction** | B1 | 2 | Independent of first B1. Statement needs to clearly intend a vertical downwards move of 3, without ambiguity or contradiction, such as '3 down', '-3 in the y direction' etc or vector notation. B0 if direction unclear, such as 'in the y-axis' (could be along or towards) or 'along the y-axis' (unless direction made clear). Allow '3' or '3 units' but not '3 places', '3 spaces', '3 squares', '3 coordinates' or mention of (scale) factor of 3. If both a valid statement and an ambiguous statement are made eg '3 units down on the y-axis' then still award B1. Ignore irrelevant statements, such as where the y-intercept is, whether correct or incorrect.

Give B0D on double negatives eg 'down the y-axis by -3 units' unless clearly wrong or contradictory eg 'negative y-direction by $\begin{pmatrix} -3 \\ 0 \end{pmatrix}$'.

**(ii)** $y = -2$ | B1 | 1 | State or imply $y = -2$. Just stating -2 is enough. B0 for final answer of $2^0 - 3$ or $1 - 3$.

(-2, 0) is B0 unless -2 already seen or implied as y-coordinate.

**(iii)** $2^x = 3$, $x = \log_2 3$ | M1 | Attempt to solve $2^x - 3 = 0$. Rearrange to $2^x = 3$, introduce logarithms (could be no base or any base as long as consistent) and then attempt expression for $x$.

M0 for $x = \log_2 2$. M1 A0 for alternative, correct, log expressions such as $\log_3 2$ or $1/\log_3 2$.

Decimal equivalent of 1.58 can get M1 A0.

$x = \log_3 (y + 3)$ is M0 (unless $y$ then becomes 0).

**State $\log_2 3$** | A1 | 2 | Doesn't need to be $x = ...$. Change of base is not on the specification, but is a valid method and can gain both marks. Allow if base not initially specified, but then both logs become base 2.

NB $x - \log_2 3 = 0$ leading to correct answer, can get full marks as there is no incorrect statement seen.

**(iv)** $2^p = 65$, $\log 2^p = \log 65$, $p\log 2 = \log 65$, $p = 6.02$ | M1* | Rearrange equation and introduce logs (or $\log_2$). Must first rearrange to $2^p = k$, with $k$ from attempt at $62 \pm 3$, before introducing logs.

**Drop power and attempt to solve** | M1d* | Dependent on first M1. If taking logs to any other base, or no base, or $\log_2$ on both sides then need to drop power of $p$ and attempt to solve using a sound algebraic method ie $p = \frac{\log k}{\log 2}$.

**Obtain 6.02, or better** | A1 | 3 | Decimal answer reqd, if more than 3sf it must be in range [6.022, 6.023]. Answer only, or trial and improvement, is 0/3 as no evidence of using logs as requested.

**(v)** $0.5 \times 0.5 \left[ 2^3 - 3 + 2(2^{3.5} - 3) + 2^4 - 3 \right] = 8.66$ | M1 | Attempt y-values at $x = 3, 3.5, 4$. M0 if other y-values also found (unless not used in trap rule). Allow M1 for using incorrect function as long as still clearly y-values that are intended to be the original function eg $2x - 3$ or $2^{(y-3)}$.

**Attempt correct trapezium rule** | M1 | Must be correct structure ie $0.5 \times 0.5 \times (y_0 + 2y_1 + y_2)$. Must be finding area from 3 to 4, so using eg $x = 0, 0.5, 1$ is M0. Allow if still in terms of $y_i$ etc as long as these have been clearly defined elsewhere. Using x-values in trapezium rule is M0, even if labelled y-values. Allow a different number of strips (except 1) as long as their $h$ is consistent with this, and the limits are still 3 and 4.

**Obtain 8.66, or better** | A1 | 3 | If final answer given to more than 3sf, allow answers in range [8.655, 8.657].

Exact answer from integrating $e^{\ln 2 - 3}$ is 0/3.

Answer only is 0/3.

Attempting integration before using trapezium rule is 0/3.

Using two separate trapezia is fine.

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8\\
\includegraphics[max width=\textwidth, alt={}, center]{4d03f4e3-ae6c-4a0d-ae4d-d89258d2919a-4_417_931_255_607}

The diagram shows the curve $y = 2 ^ { x } - 3$.\\
(i) Describe the geometrical transformation that transforms the curve $y = 2 ^ { x }$ to the curve $y = 2 ^ { x } - 3$.\\
(ii) State the $y$-coordinate of the point where the curve $y = 2 ^ { x } - 3$ crosses the $y$-axis.\\
(iii) Find the $x$-coordinate of the point where the curve $y = 2 ^ { x } - 3$ crosses the $x$-axis, giving your answer in the form $\log _ { a } b$.\\
(iv) The curve $y = 2 ^ { x } - 3$ passes through the point ( $p , 62$ ). Use logarithms to find the value of $p$, correct to 3 significant figures.\\
(v) Use the trapezium rule, with 2 strips each of width 0.5 , to find an estimate for $\int _ { 3 } ^ { 4 } \left( 2 ^ { x } - 3 \right) \mathrm { d } x$. Give your answer correct to 3 significant figures.

\hfill \mbox{\textit{OCR C2 2011 Q8 [11]}}