Edexcel C34 2019 June — Question 14 7 marks

Exam BoardEdexcel
ModuleC34 (Core Mathematics 3 & 4)
Year2019
SessionJune
Marks7
PaperDownload PDF ↗
Mark schemeDownload PDF ↗
TopicExponential Functions
TypeFind intersection of exponential curves
DifficultyModerate -0.5 Part (a) requires simple substitution of x=0 into both equations and finding the distance between two points on the y-axis. Part (b) involves setting the equations equal, which leads to a straightforward algebraic manipulation with exponentials that can be solved by substitution (letting u = e^x). This is a routine C3/C4 question testing basic exponential manipulation with no novel insight required, making it slightly easier than average.
Spec1.02q Use intersection points: of graphs to solve equations1.06a Exponential function: a^x and e^x graphs and properties1.06b Gradient of e^(kx): derivative and exponential model1.06f Laws of logarithms: addition, subtraction, power rules

14. \begin{figure}[h]
\includegraphics[alt={},max width=\textwidth]{a9870c94-0910-46ec-a54a-44a431cb324e-46_524_855_255_539} \captionsetup{labelformat=empty} \caption{Figure 5}
\end{figure} Figure 5 shows a sketch of the curves \(C _ { 1 }\) and \(C _ { 2 }\) $$\begin{aligned} & C _ { 1 } \text { has equation } y = 3 + \mathrm { e } ^ { x + 1 } \quad x \in \mathbb { R } \\ & C _ { 2 } \text { has equation } y = 10 - \mathrm { e } ^ { x } \quad x \in \mathbb { R } \end{aligned}$$ Given that \(C _ { 1 }\) and \(C _ { 2 }\) cut the \(y\)-axis at the points \(P\) and \(Q\) respectively,
  1. find the exact distance \(P Q\). \(C _ { 1 }\) and \(C _ { 2 }\) intersect at the point \(R\).
  2. Find the exact coordinates of \(R\).
    VIIIV SIHI NI IAIUM ION OCVIIV SIHI NI JIIIM ION OCVIIV SIHI NI JIIYM ION OC

Question 14(a):
AnswerMarks Guidance
Answer/WorkingMark Guidance
\(y\)-intercepts: \(9\) or \(3+e\)M1 For sight of either intercept \(9\) (not \(10-e^0\)) or \(3+e\) or \(3+e^1\) or \(3+e^{0+1}\)
Distance \(PQ = 6-e\)A1 \(6-e\) (Not \(9-(3+e)\))
Question 14(b):
AnswerMarks Guidance
Answer/WorkingMark Guidance
Sets \(3+e^{x+1} = 10-e^x\)M1 Equates the 2 curves
\(e^x(e+1) = 7\)M1 Collects exponential terms and takes out factor of \(e^x\) with correct index work
\(x = \ln\left(\frac{7}{1+e}\right)\) or \(\ln\frac{7}{1+e}\)A1 Correct \(x\)-coordinate
Substitutes \(x = \ln\left(\frac{7}{1+e}\right)\) into \(y = 10-e^x \Rightarrow y = \ldots\)ddM1 Dependent on both M's. Substituting their \(x\) into either equation to find \(y\)
\(R = \left(\ln\left(\frac{7}{1+e}\right), \frac{3+10e}{1+e}\right)\)A1 Equivalent forms: \(x=\ln\left(\frac{7}{1+e}\right)\), \(y=10-\frac{7}{1+e}\) or \(y=3+\frac{7e}{1+e}\) but not \(y=10-e^{\ln\frac{7}{1+e}}\)
Way 2:
AnswerMarks Guidance
Answer/WorkingMark Guidance
\(y=10-e^x \Rightarrow e^x=10-y \Rightarrow x=\ln(10-y) \Rightarrow y=3+e^{1+\ln(10-y)}\); makes \(x\) subject of equation 2 and substitutes into equation 1M1
\(y = 3+(10-y)e\)M1 Uses correct index work to eliminate the "ln"
\(y = \frac{10e+3}{1+e}\) or \(10-\frac{7}{1+e}\) or \(y=3+\frac{7e}{1+e}\)A1 Correct \(y\)-coordinate. Not \(y=10-e^{\ln\frac{7}{1+e}}\)
\(x = \ln(10-y) = \ln\left(10-\frac{10e+3}{1+e}\right)\)ddM1 Dependent on both M's. Substituting their \(y\) into either equation to find \(x\)
\(R = \left(\ln\left(\frac{7}{1+e}\right), \frac{3+10e}{1+e}\right)\)A1 Allow \(x=\ln\left(10-\frac{10e+3}{1+e}\right)\)
Way 3:
AnswerMarks Guidance
Answer/WorkingMark Guidance
\(y=3+e^{x+1} \Rightarrow y-3=e^{x+1} \Rightarrow e^x=\frac{y-3}{e} \Rightarrow 10-y=\frac{y-3}{e}\); makes \(e^x\) subject of equation 1 and substitutes into equation 2M1
\(10e-ye=y-3 \Rightarrow y(1+e)=10e+3\)M1 Uses correct algebra and factorises \(y\)
\(y=\frac{10e+3}{1+e}\) or \(10-\frac{7}{1+e}\) or \(y=3+\frac{7e}{1+e}\)A1 Correct \(y\)-coordinate
Then as Way 1 aboveddM1, A1
Way 4:
AnswerMarks Guidance
Answer/WorkingMark Guidance
\(y=10-e^x \Rightarrow x=\ln(10-y)\); \(y=3+e^{x+1} \Rightarrow \ln(10-y)=\ln(y-3)-1\); makes \(x\) and \(x+1\) the subject and uses \(x=x\)M1
\(\frac{y-3}{10-y}=e\)M1 Uses correct work to eliminate the "ln"s
\(y=\frac{10e+3}{1+e}\) or \(10-\frac{7}{1+e}\) or \(y=3+\frac{7e}{1+e}\)A1 Correct \(y\)-coordinate
Then as aboveddM1, A1
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## Question 14(a):

| Answer/Working | Mark | Guidance |
|---|---|---|
| $y$-intercepts: $9$ or $3+e$ | M1 | For sight of either intercept $9$ (not $10-e^0$) or $3+e$ or $3+e^1$ or $3+e^{0+1}$ |
| Distance $PQ = 6-e$ | A1 | $6-e$ (Not $9-(3+e)$) |

---

## Question 14(b):

| Answer/Working | Mark | Guidance |
|---|---|---|
| Sets $3+e^{x+1} = 10-e^x$ | M1 | Equates the 2 curves |
| $e^x(e+1) = 7$ | M1 | Collects exponential terms and takes out factor of $e^x$ with correct index work |
| $x = \ln\left(\frac{7}{1+e}\right)$ or $\ln\frac{7}{1+e}$ | A1 | Correct $x$-coordinate |
| Substitutes $x = \ln\left(\frac{7}{1+e}\right)$ into $y = 10-e^x \Rightarrow y = \ldots$ | ddM1 | Dependent on both M's. Substituting their $x$ into either equation to find $y$ |
| $R = \left(\ln\left(\frac{7}{1+e}\right), \frac{3+10e}{1+e}\right)$ | A1 | Equivalent forms: $x=\ln\left(\frac{7}{1+e}\right)$, $y=10-\frac{7}{1+e}$ or $y=3+\frac{7e}{1+e}$ but **not** $y=10-e^{\ln\frac{7}{1+e}}$ |

**Way 2:**

| Answer/Working | Mark | Guidance |
|---|---|---|
| $y=10-e^x \Rightarrow e^x=10-y \Rightarrow x=\ln(10-y) \Rightarrow y=3+e^{1+\ln(10-y)}$; makes $x$ subject of equation 2 and substitutes into equation 1 | M1 | |
| $y = 3+(10-y)e$ | M1 | Uses correct index work to eliminate the "ln" |
| $y = \frac{10e+3}{1+e}$ or $10-\frac{7}{1+e}$ or $y=3+\frac{7e}{1+e}$ | A1 | Correct $y$-coordinate. **Not** $y=10-e^{\ln\frac{7}{1+e}}$ |
| $x = \ln(10-y) = \ln\left(10-\frac{10e+3}{1+e}\right)$ | ddM1 | Dependent on both M's. Substituting their $y$ into either equation to find $x$ |
| $R = \left(\ln\left(\frac{7}{1+e}\right), \frac{3+10e}{1+e}\right)$ | A1 | Allow $x=\ln\left(10-\frac{10e+3}{1+e}\right)$ |

**Way 3:**

| Answer/Working | Mark | Guidance |
|---|---|---|
| $y=3+e^{x+1} \Rightarrow y-3=e^{x+1} \Rightarrow e^x=\frac{y-3}{e} \Rightarrow 10-y=\frac{y-3}{e}$; makes $e^x$ subject of equation 1 and substitutes into equation 2 | M1 | |
| $10e-ye=y-3 \Rightarrow y(1+e)=10e+3$ | M1 | Uses correct algebra and factorises $y$ |
| $y=\frac{10e+3}{1+e}$ or $10-\frac{7}{1+e}$ or $y=3+\frac{7e}{1+e}$ | A1 | Correct $y$-coordinate |
| Then as Way 1 above | ddM1, A1 | |

**Way 4:**

| Answer/Working | Mark | Guidance |
|---|---|---|
| $y=10-e^x \Rightarrow x=\ln(10-y)$; $y=3+e^{x+1} \Rightarrow \ln(10-y)=\ln(y-3)-1$; makes $x$ and $x+1$ the subject and uses $x=x$ | M1 | |
| $\frac{y-3}{10-y}=e$ | M1 | Uses correct work to eliminate the "ln"s |
| $y=\frac{10e+3}{1+e}$ or $10-\frac{7}{1+e}$ or $y=3+\frac{7e}{1+e}$ | A1 | Correct $y$-coordinate |
| Then as above | ddM1, A1 | |

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14.

\begin{figure}[h]
\begin{center}
  \includegraphics[alt={},max width=\textwidth]{a9870c94-0910-46ec-a54a-44a431cb324e-46_524_855_255_539}
\captionsetup{labelformat=empty}
\caption{Figure 5}
\end{center}
\end{figure}

Figure 5 shows a sketch of the curves $C _ { 1 }$ and $C _ { 2 }$

$$\begin{aligned}
& C _ { 1 } \text { has equation } y = 3 + \mathrm { e } ^ { x + 1 } \quad x \in \mathbb { R } \\
& C _ { 2 } \text { has equation } y = 10 - \mathrm { e } ^ { x } \quad x \in \mathbb { R }
\end{aligned}$$

Given that $C _ { 1 }$ and $C _ { 2 }$ cut the $y$-axis at the points $P$ and $Q$ respectively,
\begin{enumerate}[label=(\alph*)]
\item find the exact distance $P Q$.\\
$C _ { 1 }$ and $C _ { 2 }$ intersect at the point $R$.
\item Find the exact coordinates of $R$.

\begin{center}
\begin{tabular}{|l|l|l|}
\hline
VIIIV SIHI NI IAIUM ION OC & VIIV SIHI NI JIIIM ION OC & VIIV SIHI NI JIIYM ION OC \\
\hline
\end{tabular}
\end{center}
\end{enumerate}

\hfill \mbox{\textit{Edexcel C34 2019 Q14 [7]}}