A Level H2 Mathematics Practice Paper 2

TuitionGoWhere Practice Paper - Maths H2 A-Level

Answer Key — Algebra & Functions (Version 2 of 5)

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Section A

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Question 1 [5]

(a) Let $y = f(x) = \dfrac{2x - 3}{x + 1}$.

Swap $x$ and $y$:
$x = \dfrac{2y - 3}{y + 1}$

$x(y + 1) = 2y - 3$
$xy + x = 2y - 3$
$xy - 2y = -3 - x$
$y(x - 2) = -3 - x$
$y = \dfrac{-3 - x}{x - 2} = \dfrac{x + 3}{2 - x}$

$\boxed{f^{-1}(x) = \dfrac{x + 3}{2 - x}}$

Domain of $f^{-1}$ = range of $f$. Since $f(x) = \dfrac{2x-3}{x+1} = 2 - \dfrac{5}{x+1}$, the horizontal asymptote is $y = 2$, so $f(x) \neq 2$.

$\boxed{\text{Domain of } f^{-1}: x \in \mathbb{R}, x \neq 2}$

Marking: M1 for swapping and rearranging, A1 for correct inverse, A1 for correct domain.

(b) Range of $f$ = domain of $f^{-1}$, so:

$\boxed{\text{Range of } f: f(x) \in \mathbb{R}, f(x) \neq 2}$

Marking: B1 (independent).

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Question 2 [5]

(a) Range of $f$: $f(x) = x^2 - 4x + 5 = (x-2)^2 + 1$, with domain $x \geq 2$.
Since $x \geq 2$, $(x-2)^2 \geq 0$, so $f(x) \geq 1$. Range of $f$ is $[1, \infty)$.

Domain of $g$ is $x \neq 3$. Since range of $f$ is $[1, \infty)$ and $3 \in [1, \infty)$, we must check: does $f(x) = 3$ for some $x \geq 2$?

$(x-2)^2 + 1 = 3 \Rightarrow (x-2)^2 = 2 \Rightarrow x = 2 \pm \sqrt{2}$. Since $x \geq 2$, $x = 2 + \sqrt{2} \geq 2$, so $f(2+\sqrt{2}) = 3$, which is not in the domain of $g$.

Therefore, $gf$ does not exist as stated — the range of $f$ is not a subset of the domain of $g$.

However, if the question intends for students to check existence: The composite $gf$ exists only if range of $f$ ⊆ domain of $g$. Since $3 \in [1,\infty)$ and $3 \notin \text{dom}(g)$, the composite does not exist without restricting the domain of $f$.

Revised interpretation (for a solvable question): If we restrict $f$ to $x \geq 2$ with $x \neq 2 + \sqrt{2}$, then $gf$ exists.

For the purpose of this answer key, assuming the question expects students to identify the issue:

$\boxed{gf \text{ does not exist because } 3 \in \text{Range}(f) \text{ but } 3 \notin \text{Domain}(g).}$

Marking: M1 for finding range of f, M1 for checking against domain of g, A1 for correct conclusion.

(b) If we proceed formally (assuming the composite is intended to exist with appropriate restriction):

$gf(x) = g(f(x)) = \dfrac{1}{f(x) - 3} = \dfrac{1}{(x-2)^2 + 1 - 3} = \dfrac{1}{(x-2)^2 - 2}$

For $x \geq 2$, $(x-2)^2 \geq 0$, so $(x-2)^2 - 2 \geq -2$. The expression is undefined when $(x-2)^2 = 2$, i.e., $x = 2 \pm \sqrt{2}$.

Range: As $(x-2)^2 \to \infty$, $gf(x) \to 0$. At $(x-2)^2 = 0$ (i.e., $x = 2$), $gf(2) = \dfrac{1}{-2} = -\dfrac{1}{2}$. The function has a vertical asymptote at $x = 2 + \sqrt{2}$.

For $2 \leq x < 2 + \sqrt{2}$: $(x-2)^2 - 2$ goes from $-2$ to $0^-$, so $gf(x)$ goes from $-\dfrac{1}{2}$ to $-\infty$. Range: $(-\infty, -\dfrac{1}{2}]$.

For $x > 2 + \sqrt{2}$: $(x-2)^2 - 2$ goes from $0^+$ to $\infty$, so $gf(x)$ goes from $+\infty$ to $0^+$. Range: $(0, \infty)$.

$\boxed{\text{Range of } gf: (-\infty, -\tfrac{1}{2}] \cup (0, \infty)}$

Marking: M1 for correct composite expression, A1 for range.

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Question 3 [5]

(a) Let $y = \ln(3x + 2)$.

$e^y = 3x + 2$
$x = \dfrac{e^y - 2}{3}$

$\boxed{f^{-1}(x) = \dfrac{e^x - 2}{3}}$

Marking: M1 for converting to exponential form, A1 for correct inverse.

(b) Domain of $f^{-1}$ = range of $f$: Since $f(x) = \ln(3x+2)$ with $x > -\dfrac{2}{3}$, as $x \to -\dfrac{2}{3}^+$, $f(x) \to -\infty$; as $x \to \infty$, $f(x) \to \infty$.

$\boxed{\text{Domain of } f^{-1}: x \in \mathbb{R}}$

Range of $f^{-1}$ = domain of $f$:

$\boxed{\text{Range of } f^{-1}: f^{-1}(x) > -\dfrac{2}{3}}$

Marking: B1 for domain, B1 for range.

(c) Graph of $y = f(x) = \ln(3x+2)$:

• Vertical asymptote at $x = -\dfrac{2}{3}$
• Passes through $(0, \ln 2)$ since $f(0) = \ln 2$
• Passes through $\left(\dfrac{1}{3}, \ln 3\right)$ since $f\left(\dfrac{1}{3}\right) = \ln 3$

Graph of $y = f^{-1}(x) = \dfrac{e^x - 2}{3}$:

• Horizontal asymptote at $y = -\dfrac{2}{3}$ as $x \to -\infty$
• Passes through $(\ln 2, 0)$ and $(\ln 3, \dfrac{1}{3})$

The two graphs are reflections of each other in the line $y = x$.

$x$-intercept of $f$: set $f(x) = 0$: $\ln(3x+2) = 0 \Rightarrow 3x + 2 = 1 \Rightarrow x = -\dfrac{1}{3}$. So $\left(-\dfrac{1}{3}, 0\right)$.

$y$-intercept of $f$: $f(0) = \ln 2$. So $(0, \ln 2)$.

$x$-intercept of $f^{-1}$: set $f^{-1}(x) = 0$: $\dfrac{e^x - 2}{3} = 0 \Rightarrow e^x = 2 \Rightarrow x = \ln 2$. So $(\ln 2, 0)$.

$y$-intercept of $f^{-1}$: $f^{-1}(0) = \dfrac{1 - 2}{3} = -\dfrac{1}{3}$. So $\left(0, -\dfrac{1}{3}\right)$.

$\boxed{x\text{-intercept of } f: \left(-\dfrac{1}{3}, 0\right); \quad y\text{-intercept of } f: (0, \ln 2)}$

Marking: B1 for correct sketch description/intercepts.

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Question 4 [2]

$f^{-1}(4)$ means the value of $x$ such that $f(x) = 4$.

$e^{2x} + 3 = 4$
$e^{2x} = 1$
$2x = 0$
$x = 0$

$\boxed{f^{-1}(4) = 0}$

Marking: M1 for setting f(x) = 4, A1 for x = 0.

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Question 5 [3]

(a) $f(x) = \sqrt{4 - x^2}$, $-2 \leq x \leq 0$.

This is the left half of a circle of radius 2 centred at the origin (since $y = \sqrt{4-x^2}$ gives the upper semicircle, and $x \leq 0$ restricts to the left half).

At $x = -2$: $f(-2) = \sqrt{4 - 4} = 0$
At $x = 0$: $f(0) = \sqrt{4} = 2$

Since $f$ is increasing on $[-2, 0]$ (as $x$ increases, $4-x^2$ increases, so $\sqrt{4-x^2}$ increases):

$\boxed{\text{Range of } f: [0, 2]}$

Marking: M1 for evaluating endpoints, A1 for correct range.

(b) Since $f$ is one-one on $[-2, 0]$ (it is strictly increasing), $f^{-1}$ exists.

Let $y = \sqrt{4 - x^2}$, $y \geq 0$.
$y^2 = 4 - x^2$
$x^2 = 4 - y^2$
$x = -\sqrt{4 - y^2}$ (negative root since $x \leq 0$)

$\boxed{f^{-1}(x) = -\sqrt{4 - x^2}}$

Domain of $f^{-1}$ = range of $f$: $\boxed{[0, 2]}$
Range of $f^{-1}$ = domain of $f$: $\boxed{[-2, 0]}$

Marking: B1 for stating inverse exists (one-one), B1 for correct inverse with domain and range.

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Section B

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Question 6 [7]

(a) $fg(x) = f(g(x)) = f\left(\dfrac{x+4}{x-1}\right) = 3\left(\dfrac{x+4}{x-1}\right) - 2 = \dfrac{3(x+4) - 2(x-1)}{x-1} = \dfrac{3x + 12 - 2x + 2}{x-1}$

$\boxed{fg(x) = \dfrac{x + 14}{x - 1}}$

The composite exists because range of $g$ is $\mathbb{R} \setminus \{3\}$ (since $g(x) = 1 + \frac{5}{x-1} \neq 3$... wait, let me recalculate).

$g(x) = \dfrac{x+4}{x-1} = 1 + \dfrac{5}{x-1}$. As $x \to 1^{\pm}$, $g(x) \to \pm \infty$. As $x \to \pm \infty$, $g(x) \to 1$. So range of $g$ is $\mathbb{R} \setminus \{1\}$.

Domain of $f$ is $\mathbb{R}$, so range of $g$ ⊆ domain of $f$. ✓

$\boxed{fg \text{ exists since Range}(g) = \mathbb{R} \setminus \{1\} \subseteq \mathbb{R} = \text{Domain}(f).}$

Marking: M1 for computing composite, A1 for simplified expression, B1 for existence justification.

(b) $fg(x) = \dfrac{x+14}{x-1} = 1 + \dfrac{15}{x-1}$

As $x \to 1^{\pm}$, $fg(x) \to \pm \infty$. As $x \to \pm \infty$, $fg(x) \to 1$. So $fg(x) \neq 1$.

$\boxed{\text{Range of } fg: fg(x) \in \mathbb{R}, fg(x) \neq 1}$

Marking: M1 for rewriting in form 1 + k/(x-1), A1 for correct range.

(c) $fg(x) = x$
$\dfrac{x + 14}{x - 1} = x$
$x + 14 = x(x - 1) = x^2 - x$
$x^2 - 2x - 14 = 0$

$x = \dfrac{2 \pm \sqrt{4 + 56}}{2} = \dfrac{2 \pm \sqrt{60}}{2} = \dfrac{2 \pm 2\sqrt{15}}{2} = 1 \pm \sqrt{15}$

$\boxed{x = 1 + \sqrt{15} \text{ or } x = 1 - \sqrt{15}}$

Marking: M1 for setting up equation, M1 for solving quadratic, A1 for both solutions.

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Question 7 [8]

(a) Let $y = \dfrac{ax + b}{cx + d}$.

$y(cx + d) = ax + b$
$cxy + dy = ax + b$
$cxy - ax = b - dy$
$x(cy - a) = b - dy$
$x = \dfrac{b - dy}{cy - a}$

$\boxed{f^{-1}(x) = \dfrac{b - dx}{cx - a}}$

Marking: M1 for cross-multiplication and rearrangement, A1 for correct inverse.

(b) For $f$ to be self-inverse: $f^{-1}(x) = f(x)$

$\dfrac{b - dx}{cx - a} = \dfrac{ax + b}{cx + d}$

Cross-multiplying: $(b - dx)(cx + d) = (ax + b)(cx - a)$

LHS: $bcx + bd - dcx^2 - d^2x = -dcx^2 + (bc - d^2)x + bd$

RHS: $acx^2 - a^2x + bcx - a^2$... let me redo:

RHS: $(ax + b)(cx - d)$... no, $(ax + b)(cx - a)$:

Wait, I should be more careful. Let me redo:

$(ax + b)(cx - d)$ — no, the denominator of $f^{-1}$ is $cx - a$ and numerator is $b - dx$.

So: $(b - dx)(cx + d) = (ax + b)(cx + d)$... no, that's not right either.

Let me restart. We need: $\frac{b - dx}{cx - a} = \frac{ax + b}{cx + d}$

Cross multiply: $(b - dx)(cx + d) = (ax + b)(cx - d)$... no, $(ax + b)(cx + d)$ is wrong too.

$(b - dx)(cx + d) = (ax + b)(cx - a)$ — no, the RHS denominator is $cx + d$ and we're cross-multiplying with $cx - a$:

$(b - dx)(cx + d) = (ax + b)(cx - a)$

LHS: $bcx + bd - dcx^2 - d^2x = -dcx^2 + (bc - d^2)x + bd$

RHS: $acx^2 - a^2x + bcx - ba = acx^2 + (bc - a^2)x - ab$

For these to be equal for all $x$:

Coefficient of $x^2$: $-dc = ac \Rightarrow c(a + d) = 0$. Since $c \neq 0$, $a + d = 0$. ✓

Coefficient of $x$: $bc - d^2 = bc - a^2 \Rightarrow d^2 = a^2$. If $a + d = 0$, then $d = -a$, so $d^2 = a^2$. ✓

Constant: $bd = -ab \Rightarrow b(d + a) = 0$. If $a + d = 0$, this is satisfied. ✓

Conversely, if $a + d = 0$ (so $d = -a$), then:

$f^{-1}(x) = \dfrac{b - (-a)x}{cx - a} = \dfrac{b + ax}{cx - a}$

And $f(x) = \dfrac{ax + b}{cx - a}$ (since $d = -a$).

So $f^{-1}(x) = f(x)$. ✓

$\boxed{f \text{ is self-inverse } \iff a + d = 0}$

Marking: M1 for setting f⁻¹ = f, M1 for cross-multiplying, M1 for comparing coefficients, A1 for deducing a + d = 0, B1 for converse.

(c) $f(x) = \dfrac{2x + 3}{x - 2}$. Here $a = 2, b = 3, c = 1, d = -2$. So $a + d = 2 + (-2) = 0$. ✓ Self-inverse verified.

Solve $f(x) = x$:
$\dfrac{2x + 3}{x - 2} = x$
$2x + 3 = x(x - 2) = x^2 - 2x$
$x^2 - 4x - 3 = 0$

$x = \dfrac{4 \pm \sqrt{16 + 12}}{2} = \dfrac{4 \pm \sqrt{28}}{2} = \dfrac{4 \pm 2\sqrt{7}}{2} = 2 \pm \sqrt{7}$

$\boxed{x = 2 + \sqrt{7} \text{ or } x = 2 - \sqrt{7}}$

Marking: B1 for verifying a + d = 0, M1 for setting f(x) = x, A1 for solutions.

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Question 8 [5]

(a) For $x \leq 1$: $f(x) = x^2 + 2x = (x+1)^2 - 1$. This is a parabola with vertex at $x = -1$, opening upwards. On $(-\infty, -1]$ it is decreasing; on $[-1, 1]$ it is increasing. So $f$ is not one-one on $x \leq 1$ (e.g., $f(-3) = 9 - 6 = 3$ and $f(1) = 1 + 2 = 3$).

For $x > 1$: $f(x) = 4 - x$, which is strictly decreasing (one-one).

Overall: $f(-3) = 3$ and $f(1) = 3$, so $f$ is not one-one on its full domain.

$\boxed{f \text{ is not one-one because, for example, } f(-3) = f(1) = 3.}$

Marking: M1 for checking each piece, A1 for correct conclusion with counterexample.

(b) For $x \leq 1$: $f(x) = (x+1)^2 - 1 \geq -1$. At $x = 1$, $f(1) = 3$. As $x \to -\infty$, $f(x) \to \infty$. Minimum is $-1$ at $x = -1$. So range for $x \leq 1$ is $[-1, \infty)$... wait, at $x=1$, $f(1) = 3$, and the function decreases from $\infty$ to $-1$ as $x$ goes from $-\infty$ to $-1$, then increases from $-1$ to $3$ as $x$ goes from $-1$ to $1$. So range for $x \leq 1$ is $[-1, \infty)$.

For $x > 1$: $f(x) = 4 - x < 3$. As $x \to 1^+$, $f(x) \to 3^-$. As $x \to \infty$, $f(x) \to -\infty$. So range for $x > 1$ is $(-\infty, 3)$.

Combined range: $[-1, \infty) \cup (-\infty, 3) = (-\infty, \infty) = \mathbb{R}$.

Wait, but $[-1, \infty) \cup (-\infty, 3) = \mathbb{R}$ since $[-1, \infty)$ already covers everything $\geq -1$ and $(-\infty, 3)$ covers everything $< 3$, and together they cover all reals.

$\boxed{\text{Range of } f: \mathbb{R}}$

Marking: M1 for finding range of each piece, A1 for combined range.

(c) For $g$ to have an inverse when restricted to $x \leq 1$, $g$ must be one-one on $x \leq 1$. But $g(x) = f(x) + k = x^2 + 2x + k$ for $x \leq 1$, which is a parabola with vertex at $x = -1$. It is not one-one on $(-\infty, 1]$ regardless of $k$ (since the vertex is at $x = -1$ and the function decreases then increases).

Re-reading the question: "State the least value of $k$ such that $g$ has an inverse function when the domain of $g$ is restricted to $x \leq 1$."

Adding a constant $k$ shifts the graph vertically but does not change the shape. The function $x^2 + 2x + k$ is never one-one on $(-\infty, 1]$ because it has a turning point at $x = -1$ within this domain.

Perhaps the question means: restrict the domain further to make it one-one, and find $k$ so that the restricted function has an inverse. But the question says "the domain of $g$ is restricted to $x \leq 1$."

Alternative interpretation: Perhaps the question intends for the domain to be restricted to a subset of $x \leq 1$ where $g$ is one-one, e.g., $[-1, 1]$ or $(-\infty, -1]$. But the wording says "restricted to $x \leq 1$."

Let me reconsider: maybe the question is asking for the least $k$ such that $g(x) = f(x) + k$ is one-one on $x \leq 1$. Since $f(x) = x^2 + 2x$ on $x \leq 1$ is not one-one (it has a minimum at $x = -1$), no value of $k$ can make it one-one.

Perhaps the question intends: restrict to $x \geq 1$ (where $f(x) = 4 - x$ is already one-one), or perhaps it's about making the function strictly monotonic.

Given the ambiguity, let me reinterpret: perhaps the question means the domain is restricted to $[m, 1]$ for some $m$ where $g$ becomes one-one, and we need the least $k$ so that...

Actually, re-reading more carefully: I think the question might mean that we restrict the domain of $g$ to $x \leq 1$ and want $g$ to be one-one on this domain. Since $g(x) = x^2 + 2x + k$ on $x \leq 1$, and this is a parabola opening upward with vertex at $x = -1$, it is decreasing on $(-\infty, -1]$ and increasing on $[-1, 1]$. It is not one-one on $(-\infty, 1]$.

Let me try a different reading: Perhaps the question wants us to restrict to a subdomain of $x \leq 1$ where $g$ IS one-one, and find the least $k$ so that the range of $g$ on this restricted domain allows an inverse. But this doesn't depend on $k$.

I think the most sensible interpretation is: The question intends for the domain to be restricted to $[-1, 1]$ (where $x^2 + 2x + k$ is increasing, hence one-one), and asks for the least $k$ such that... but the question doesn't specify what condition $k$ needs to satisfy.

Let me try yet another interpretation: Perhaps the question is about making $g$ one-one on $x \leq 1$ by ensuring the function values don't repeat. But for a parabola, values always repeat (except at the vertex).

Given the difficulty, let me assume the question intends: "State the least value of $k$ such that $g$ has an inverse function when the domain of $g$ is restricted to $[-1, \infty)$" or similar. But I should answer what's written.

Simplest reasonable interpretation: The function $g(x) = x^2 + 2x + k$ on $x \leq 1$ is not one-one for any $k$. However, if we restrict to $[-1, 1]$ (a subset of $x \leq 1$), it IS one-one (increasing). The question might be poorly worded and intends this.

Alternatively: Perhaps the question means: find the least $k$ such that $g$ is one-one on $x \leq 1$. Since this is impossible for any $k$, the answer might be "no such $k$ exists."

But that seems unlikely for a 5-mark question. Let me try one more interpretation:

Perhaps the question is: restrict the domain of $g$ to $x \leq 1$, and find the least $k$ such that $g$ maps this domain to a set where an inverse can be defined (i.e., $g$ is one-one on this domain). Since $g(x) = x^2 + 2x + k$ is not one-one on $(-\infty, 1]$, no such $k$ exists.

*I'll go with the interpretation that the domain is restricted to $[-1, 1]$ (the part of $x \leq 1$ where $g$ is increasing and hence one-one), and the question asks for the least $k$ such that... hmm, but there's no condition on $k$ for one-one-ness on $[-1, 1]$.

Let me just answer based on the most natural reading I can find:

If the domain is restricted to $[-1, 1] \subseteq (-\infty, 1]$, then $g(x) = x^2 + 2x + k$ is increasing (since the vertex is at $x = -1$ and the parabola opens upward), hence one-one. The least value of $k$ is not constrained by the one-one condition... unless there's an implicit condition I'm missing.

Perhaps the question is about the range: For $g$ to have an inverse, it must be one-one. On $[-1, 1]$, $g$ is one-one for all $k$. So perhaps the question is asking for something else.

Given the time I've spent, let me provide the most reasonable answer:

$\boxed{\text{No value of } k \text{ makes } g \text{ one-one on } (-\infty, 1] \text{ since } g(x) = x^2 + 2x + k \text{ has a turning point at } x = -1 \text{ in this domain.}}$

If the intended domain restriction is $[-1, 1]$ instead, then $g$ is one-one for all $k$, so there is no least $k$ constraint from the one-one condition alone.

Marking: M1 for analyzing one-one property, A1 for correct conclusion.

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Question 9 [5]

(a) Let $y = f(x) = 2 + \sqrt{x - 1}$, $x \geq 1$.

$y - 2 = \sqrt{x - 1}$
$(y - 2)^2 = x - 1$
$x = (y - 2)^2 + 1$

$\boxed{f^{-1}(x) = (x - 2)^2 + 1}$

Domain of $f^{-1}$ = range of $f$: Since $x \geq 1$, $\sqrt{x-1} \geq 0$, so $f(x) \geq 2$.

$\boxed{\text{Domain of } f^{-1}: x \geq 2}$

Range of $f^{-1}$ = domain of $f$:

$\boxed{\text{Range of } f^{-1}: f^{-1}(x) \geq 1}$

Marking: M1 for squaring and rearranging, A1 for correct inverse, B1 for domain, B1 for range.

(c) The graphs of $y = f(x)$ and $y = f^{-1}(x)$ intersect on the line $y = x$ (if they intersect at all, since both are reflections of each other in $y = x$).

Set $f(x) = x$:
$2 + \sqrt{x - 1} = x$
$\sqrt{x - 1} = x - 2$

For this to be valid, $x - 2 \geq 0$, i.e., $x \geq 2$.

Squaring: $x - 1 = (x - 2)^2 = x^2 - 4x + 4$
$x^2 - 5x + 5 = 0$

$x = \dfrac{5 \pm \sqrt{25 - 20}}{2} = \dfrac{5 \pm \sqrt{5}}{2}$

Check $x \geq 2$: $\dfrac{5 - \sqrt{5}}{2} \approx \dfrac{5 - 2.236}{2} \approx 1.382 < 2$. Reject.

$\dfrac{5 + \sqrt{5}}{2} \approx 3.618 \geq 2$. ✓

Check: $f\left(\dfrac{5+\sqrt{5}}{2}\right) = 2 + \sqrt{\dfrac{5+\sqrt{5}}{2} - 1} = 2 + \sqrt{\dfrac{3+\sqrt{5}}{2}}$

And $\dfrac{5+\sqrt{5}}{2} - 2 = \dfrac{1+\sqrt{5}}{2}$, so $\sqrt{x-1} = \sqrt{\dfrac{3+\sqrt{5}}{2}}$.

Hmm, let me verify: $\left(\dfrac{1+\sqrt{5}}{2}\right)^2 = \dfrac{1 + 2\sqrt{5} + 5}{4} = \dfrac{6 + 2\sqrt{5}}{4} = \dfrac{3+\sqrt{5}}{2}$. ✓

So $f(x) = 2 + \dfrac{1+\sqrt{5}}{2} = \dfrac{5+\sqrt{5}}{2} = x$. ✓

$\boxed{\text{Point of intersection: } \left(\dfrac{5 + \sqrt{5}}{2}, \dfrac{5 + \sqrt{5}}{2}\right)}$

Marking: M1 for setting f(x) = x, M1 for solving, A1 for correct answer (rejecting extraneous root).

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Section C

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Question 10 [7]

(a) $C(t) = \dfrac{5t}{t^2 + 1}$, $t \geq 0$.

Using the quotient rule:
$C'(t) = \dfrac{5(t^2+1) - 5t(2t)}{(t^2+1)^2} = \dfrac{5t^2 + 5 - 10t^2}{(t^2+1)^2} = \dfrac{5 - 5t^2}{(t^2+1)^2} = \dfrac{5(1-t^2)}{(t^2+1)^2}$

Set $C'(t) = 0$: $1 - t^2 = 0 \Rightarrow t = 1$ (since $t \geq 0$).

Check: For $0 < t < 1$, $C'(t) > 0$ (increasing). For $t > 1$, $C'(t) < 0$ (decreasing). So $t = 1$ is a maximum.

$C(1) = \dfrac{5(1)}{1 + 1} = \dfrac{5}{2} = 2.5$

$\boxed{C_{\max} = 2.5 \text{ mol/L at } t = 1 \text{ hour}}$

Marking: M1 for differentiating, A1 for critical point, M1 for confirming maximum, A1 for values.

(b) At $t = 0$: $C(0) = 0$.
As $t \to \infty$: $C(t) = \dfrac{5t}{t^2+1} \to 0$.

Maximum is $2.5$ at $t = 1$. The function increases from $0$ to $2.5$ on $[0, 1]$ and decreases from $2.5$ to $0$ on $[1, \infty)$.

$\boxed{\text{Range of } C: [0, 2.5]}$

Marking: M1 for evaluating endpoints and limit, A1 for correct range.

(c) $C$ is not one-one on $t \geq 0$ because it increases on $[0, 1]$ and decreases on $[1, \infty)$. For example, $C(0) = 0$ and $\lim_{t \to \infty} C(t) = 0$, and by continuity, every value in $(0, 2.5)$ is attained twice.

Therefore, $C^{-1}$ does not exist for domain $t \geq 0$.

To make $C^{-1}$ exist, restrict the domain to either $[0, 1]$ (where $C$ is strictly increasing) or $[1, \infty)$ (where $C$ is strictly decreasing).

Restriction to $[0, 1]$: Let $y = \dfrac{5t}{t^2 + 1}$.

$y(t^2 + 1) = 5t$
$yt^2 - 5t + y = 0$

$t = \dfrac{5 \pm \sqrt{25 - 4y^2}}{2y}$ (for $y \neq 0$)

For $t \in [0, 1]$, we need the smaller root (since $C$ is increasing on $[0,1]$):

$t = \dfrac{5 - \sqrt{25 - 4y^2}}{2y}$

$\boxed{C^{-1}(y) = \dfrac{5 - \sqrt{25 - 4y^2}}{2y}, \quad 0 < y \leq 2.5}$

Marking: M1 for explaining not one-one, M1 for suggesting restriction, M1 for finding inverse, A1 for correct expression.

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Question 11 [8]

(a) Range of $g$: $g(x) = \ln(x + 4)$, $x > -4$. As $x \to -4^+$, $g(x) \to -\infty$. As $x \to \infty$, $g(x) \to \infty$. So range of $g$ is $\mathbb{R}$.

Domain of $f$ is $\mathbb{R}$. Since range of $g$ = $\mathbb{R}$ ⊆ domain of $f$ = $\mathbb{R}$, $fg$ exists. ✓

Range of $f$: $f(x) = e^x - 1$, $x \in \mathbb{R}$. As $x \to -\infty$, $f(x) \to -1$. As $x \to \infty$, $f(x) \to \infty$. So range of $f$ is $(-1, \infty)$.

Domain of $g$ is $x > -4$. Since range of $f$ is $(-1, \infty)$ and $-1 > -4$, we have $(-1, \infty) \subseteq (-4, \infty)$. So $gf$ exists. ✓

$\boxed{fg \text{ exists since Range}(g) = \mathbb{R} \subseteq \text{Domain}(f) = \mathbb{R}; \quad gf \text{ exists since Range}(f) = (-1, \infty) \subseteq (-4, \infty) = \text{Domain}(g).}$

Marking: M1 for checking range of g against domain of f, M1 for checking range of f against domain of g, A1 for both conclusions.

(b) $fg(x) = f(g(x)) = f(\ln(x+4)) = e^{\ln(x+4)} - 1 = x + 4 - 1 = x + 3$

$\boxed{fg(x) = x + 3, \quad x > -4}$

$gf(x) = g(f(x)) = g(e^x - 1) = \ln(e^x - 1 + 4) = \ln(e^x + 3)$

$\boxed{gf(x) = \ln(e^x + 3), \quad x \in \mathbb{R}}$

Marking: A1 for fg, A1 for gf.

(c) $fg(x) = gf(x)$
$x + 3 = \ln(e^x + 3)$

Let $u = x + 3$, so $x = u - 3$:
$u = \ln(e^{u-3} + 3)$
$e^u = e^{u-3} + 3$
$e^u - e^{u-3} = 3$
$e^{u-3}(e^3 - 1) = 3$
$e^{u-3} = \dfrac{3}{e^3 - 1}$
$u - 3 = \ln\left(\dfrac{3}{e^3 - 1}\right) = \ln 3 - \ln(e^3 - 1)$
$u = 3 + \ln 3 - \ln(e^3 - 1)$
$x = u - 3 = \ln 3 - \ln(e^3 - 1) = \ln\left(\dfrac{3}{e^3 - 1}\right)$

$e^3 \approx 20.0855$
$e^3 - 1 \approx 19.0855$
$\dfrac{3}{19.0855} \approx 0.15718$
$x = \ln(0.15718) \approx -1.850$

$\boxed{x \approx -1.85 \text{ (3 s.f.)}}$

Marking: M1 for setting fg = gf, M1 for substitution, M1 for solving, A1 for correct answer.

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Mark Summary

Q	Marks
1	5
2	5
3	5
4	2
5	3
Section A	20
6	7
7	8
8	5
9	5
Section B	25
10	7
11	8
Section C	15
Total	60

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