Secondary 3 Additional Mathematics Practice Paper 5

TuitionGoWhere Practice Paper Answer Key - Additional Mathematics Secondary 3

Version 5 of 5

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Section A: Pure Mathematics

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Question 1 [3 marks]

Method: Completing the square

$f(x) = 2x^2 - 12x + 5$

Factor out 2 from the $x$ terms: $= 2(x^2 - 6x) + 5$

Complete the square inside the bracket. Half of $-6$ is $-3$, and $(-3)^2 = 9$: $= 2[(x-3)^2 - 9] + 5$ $= 2(x-3)^2 - 18 + 5$ $= 2(x-3)^2 - 13$

Answer: $f(x) = 2(x-3)^2 - 13$ [2 marks for correct completion]

Turning point: $(3, -13)$ [1 mark]

Teaching note: The form $a(x-h)^2 + k$ reveals the vertex directly as $(h, k)$. When $a > 0$, this is a minimum point.

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Question 2 [4 marks]

For no real roots, discriminant $\Delta < 0$.

Step 1: Identify coefficients: $a = 1$, $b = (k+2)$, $c = (2k+5)$

Step 2: Discriminant: $\Delta = (k+2)^2 - 4(1)(2k+5)$ $= k^2 + 4k + 4 - 8k - 20$ $= k^2 - 4k - 16$

Step 3: Require $\Delta < 0$: $k^2 - 4k - 16 < 0$

Step 4: Solve equality $k^2 - 4k - 16 = 0$: $k = \frac{4 \pm \sqrt{16 + 64}}{2} = \frac{4 \pm \sqrt{80}}{2} = \frac{4 \pm 4\sqrt{5}}{2} = 2 \pm 2\sqrt{5}$

Or numerically: $k \approx 2 \pm 4.472$, so $k \approx 6.472$ or $k \approx -2.472$

Step 5: Since coefficient of $k^2$ is positive, $k^2 - 4k - 16 < 0$ between the roots:

Answer: $2 - 2\sqrt{5} < k < 2 + 2\sqrt{5}$ [4 marks]

(Accept exact form or approximately $-2.47 < k < 6.47$)

Marking breakdown:

• Correct discriminant expression: [1 mark]
• Simplified discriminant in terms of k: [1 mark]
• Correct roots of equality: [1 mark]
• Correct inequality with proper interval: [1 mark]

Common error: Forgetting to flip the inequality or writing $k > \text{smaller root}$ and $k < \text{larger root}$ incorrectly.

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Question 3 [4 marks]

Using turning point form:

Since turning point is at $x = 4$, write $y = (x-4)^2 + q'$ for some constant $q'$.

Actually, with leading coefficient 1: $y = (x-4)^2 + c'$

Expanding: $y = x^2 - 8x + 16 + c'$

So $p = -8$.

Using point $(2, -3)$: $-3 = (2)^2 + (-8)(2) + q$ $-3 = 4 - 16 + q$ $-3 = -12 + q$ $q = 9$

Alternative method using calculus: $\frac{dy}{dx} = 2x + p = 0$ at $x = 4$, so $p = -8$. Then proceed as above.

Answer: $p = -8$, $q = 9$ [2 marks each]

Teaching note: The turning point of $y = x^2 + px + q$ occurs at $x = -\frac{p}{2}$. This is a key formula derived from completing the square.

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Question 4 [5 marks]

Step 1: Rearrange (don't multiply by $(x+3)$ without considering sign): $\frac{2x-1}{x+3} - 1 \geq 0$ $\frac{2x-1 - (x+3)}{x+3} \geq 0$ $\frac{x-4}{x+3} \geq 0$

Step 2: Critical values: $x = 4$ (numerator zero) and $x = -3$ (denominator zero, excluded)

Step 3: Sign analysis or test regions:

Region	$x-4$	$x+3$	$\frac{x-4}{x+3}$
$x < -3$	$-$	$-$	$+$
$-3 < x < 4$	$-$	$+$	$-$
$x > 4$	$+$	$+$	$+$

We need $\geq 0$, so positive regions including where numerator is zero.

Step 4: Solution excludes $x = -3$ (undefined), includes $x = 4$:

Answer: $x < -3$ or $x \geq 4$ [5 marks]

Marking breakdown:

• Correct rearrangement to single fraction: [2 marks]
• Correct critical values identified: [1 mark]
• Valid method (sign diagram, test values, or logical cases): [1 mark]
• Correct final answer with proper inequality notation: [1 mark]

Common error: Multiplying both sides by $(x+3)$ without squaring — this fails when $x+3 < 0$.

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Question 5 [4 marks]

(a) Finding $f^{-1}$ [2 marks]

Let $y = 3x - 2$

Swap and solve: $x = 3y - 2$

So $y = \frac{x+2}{3}$

Answer: $f^{-1}(x) = \frac{x+2}{3}$

Domain of $f^{-1}$ = Range of $f$ = $\mathbb{R}$ (all real numbers) [1 mark for formula, 1 mark for domain]

(b) Solving $g(x) = 2$ [2 marks]

$\frac{x+1}{x-2} = 2$

$x + 1 = 2(x-2) = 2x - 4$

$1 + 4 = 2x - x$

$x = 5$

Check: $g(5) = \frac{6}{3} = 2$ ✓

Answer: $x = 5$

Teaching note: For rational functions, always check that your answer doesn't make the denominator zero.

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Question 6 [5 marks]

From $2x^2 - 5x + 1 = 0$: $\alpha + \beta = \frac{5}{2}, \quad \alpha\beta = \frac{1}{2}$

(a) [3 marks]

$\frac{1}{\alpha^2} + \frac{1}{\beta^2} = \frac{\alpha^2 + \beta^2}{\alpha^2\beta^2} = \frac{(\alpha+\beta)^2 - 2\alpha\beta}{(\alpha\beta)^2}$

Numerator: $\left(\frac{5}{2}\right)^2 - 2 \times \frac{1}{2} = \frac{25}{4} - 1 = \frac{21}{4}$

Denominator: $\left(\frac{1}{2}\right)^2 = \frac{1}{4}$

Result: $\frac{21/4}{1/4} = 21$

Answer: $21$

Marking breakdown:

• Correct sum and product of roots: [1 mark]
• Correct expansion of $\alpha^2 + \beta^2$: [1 mark]
• Final answer: [1 mark]

(b) [2 marks]

Roots are $\alpha^2\beta$ and $\alpha\beta^2 = \alpha\beta(\alpha)$ and $\alpha\beta(\beta)$

Sum: $\alpha^2\beta + \alpha\beta^2 = \alpha\beta(\alpha + \beta) = \frac{1}{2} \times \frac{5}{2} = \frac{5}{4}$

Product: $\alpha^2\beta \times \alpha\beta^2 = \alpha^3\beta^3 = (\alpha\beta)^3 = \frac{1}{8}$

Equation: $x^2 - (\text{sum})x + (\text{product}) = 0$

Multiply by 8: $8x^2 - 10x + 1 = 0$

Answer: $8x^2 - 10x + 1 = 0$ (or equivalent)

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Question 7 [4 marks]

From the graph description:

• Maximum value = 3, minimum value = -1
• Amplitude $a = \frac{3-(-1)}{2} = 2$ [1 mark]
• Vertical shift $c = \frac{3+(-1)}{2} = 1$ [1 mark]
• Period = $\pi$ (two complete cycles in $2\pi$), so $\frac{2\pi}{b} = \pi$, giving $b = 2$ [2 marks]

Answer: $a = 2$, $b = 2$, $c = 1$ [marks as indicated]

Teaching note: For $y = a\sin(bx) + c$, amplitude = $|a|$, period = \frac{2\pi}{|b|, vertical shift = $c$ (midline).

Expected visual: The graph shows two complete sine waves starting at $(0,1)$, reaching max at $(\frac{\pi}{4}, 3)$, minimum at $(\frac{3\pi}{4}, -1)$, with period $\pi$.

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Question 8 [5 marks]

Using Remainder Theorem:

$p(1) = 7$: $2 + a + b + 3 = 7$, so $a + b = 2$ ... (1) [1 mark]

$p\left(-\frac{1}{2}\right) = 0$: $2\left(-\frac{1}{2}\right)^3 + a\left(-\frac{1}{2}\right)^2 + b\left(-\frac{1}{2}\right) + 3 = 0$ $2 \times \left(-\frac{1}{8}\right) + \frac{a}{4} - \frac{b}{2} + 3 = 0$ $-\frac{1}{4} + \frac{a}{4} - \frac{b}{2} + 3 = 0$

Multiply by 4: $-1 + a - 2b + 12 = 0$, so $a - 2b = -11$ ... (2) [2 marks]

From (1): $a = 2 - b$. Substitute: $(2-b) - 2b = -11$ $2 - 3b = -11$ $b = \frac{13}{3}$, so $a = 2 - \frac{13}{3} = -\frac{7}{3}$

Wait — let me recheck: $2 - 3b = -11$ gives $-3b = -13$, so $b = \frac{13}{3}$... This seems messy. Let me recheck equation (2).

Actually: $-\frac{1}{4} + \frac{a}{4} - \frac{b}{2} + 3 = 0$

Multiply by 4: $-1 + a - 2b + 12 = 0$, so $a - 2b = -11$. Yes.

From (1): $a = 2 - b$. Then: $2 - b - 2b = -11$, so $2 - 3b = -11$, thus $b = \frac{13}{3}$.

Hmm, this gives non-integer answer. Let me recheck $p(-\frac{1}{2})$:

$2 \times (-\frac{1}{8}) = -\frac{1}{4}$ ✓

Actually, let me verify if the problem should have $2x-1$ as factor. But problem states $2x+1$.

Rechecking arithmetic: $-\frac{1}{4} + 3 = \frac{11}{4}$, so $\frac{a}{4} - \frac{b}{2} = -\frac{11}{4}$, giving $a - 2b = -11$. Yes.

So $a = -\frac{7}{3}$, $b = \frac{13}{3}$. This is valid but unusual. Let me proceed — or recheck if I made an error.

Actually, $a + b = 2$ and $a - 2b = -11$: subtracting: $3b = 13$, $b = \frac{13}{3}$, $a = -\frac{7}{3}$.

Proceeding with factorisation (noting this is an unusual but valid case):

$p(x) = 2x^3 - \frac{7}{3}x^2 + \frac{13}{3}x + 3$

With $2x+1$ as factor, and messy coefficients... Actually, let me recheck original problem setup. Perhaps let me verify remainder at $x=1$:

$p(1) = 2 + a + b + 3 = 5 + a + b = 7$, so $a + b = 2$. ✓

Given that exact factorisation may be complex, let me use polynomial division or synthetic division with root $x = -\frac{1}{2}$.

Actually, for clean answers, perhaps I should recheck. The product of roots from $p(x) = 2x^3 + ... + 3$ gives $\alpha\beta\gamma = -\frac{3}{2}$ if leading coefficient matters...

Let me just verify my arithmetic once more. The factor is $(2x+1)$, so root is $x = -\frac{1}{2}$.

$p(-\frac{1}{2}) = 2(-\frac{1}{8}) + a(\frac{1}{4}) + b(-\frac{1}{2}) + 3 = -\frac{1}{4} + \frac{a}{4} - \frac{b}{2} + 3$

$= \frac{11}{4} + \frac{a - 2b}{4} = 0$

So $11 + a - 2b = 0$, meaning $a - 2b = -11$. ✓

Solving: $a = 2-b$, so $2-b-2b = -11$, $2-3b = -11$, $b = \frac{13}{3}$.

This is correct. The answer involves fractions. Proceeding:

$p(x) = 2x^3 - \frac{7}{3}x^2 + \frac{13}{3}x + 3 = \frac{1}{3}(6x^3 - 7x^2 + 13x + 9)$

Testing: does $2x+1$ divide this? At $x = -\frac{1}{2}$: $6(-\frac{1}{8}) - 7(\frac{1}{4}) + 13(-\frac{1}{2}) + 9 = -\frac{6}{8} - \frac{7}{4} - \frac{13}{2} + 9 = -\frac{3}{4} - \frac{7}{4} - \frac{26}{4} + \frac{36}{4} = 0$. ✓

Using polynomial division or factor theorem: $(2x+1)(3x^2 - 5x + 9)/3$... Actually let's do division properly.

$6x^3 - 7x^2 + 13x + 9$ divided by $(2x+1)$:

• $3x^2$ times: $6x^3 + 3x^2$, subtract: $-10x^2 + 13x + 9$
• $-5x$ times: $-10x^2 - 5x$, subtract: $18x + 9$
• $9$ times: $18x + 9$, remainder 0.

So $6x^3 - 7x^2 + 13x + 9 = (2x+1)(3x^2 - 5x + 9)$

Check discriminant of $3x^2 - 5x + 9$: $25 - 108 = -83 < 0$, so no real factors.

Answer: $a = -\frac{7}{3}$, $b = \frac{13}{3}$; $p(x) = \frac{1}{3}(2x+1)(3x^2 - 5x + 9)$

Or equivalently: $p(x) = (2x+1)\left(x^2 - \frac{5}{3}x + 3\right)$ [5 marks]

Note: This problem illustrates that not all textbook problems yield integers; exam problems sometimes have fractional answers.

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Question 9 [6 marks]

(a) [3 marks]

$\frac{x^2 + 3}{x-1}$

Long division: $x^2 + 0x + 3$ divided by $x - 1$

• $x$ times: $x^2 - x$, subtract: $x + 3$
• $+1$ times: $x - 1$, subtract: $4$

Answer: $y = x + 1 + \frac{4}{x-1}$ [3 marks]

(b) [2 marks]

As $x \to \infty$, $\frac{4}{x-1} \to 0$, so $y \to x + 1$

Oblique asymptote: $y = x + 1$ [1 mark]

Vertical asymptote where denominator zero: $x = 1$ [1 mark]

(c) [1 mark]

For intersection with oblique asymptote: $x + 1 + \frac{4}{x-1} = x + 1$

This gives $\frac{4}{x-1} = 0$, which has no solution since numerator $4 \neq 0$.

Answer: The curve never intersects its oblique asymptote because the remainder term $\frac{4}{x-1}$ can never equal zero. [1 mark]

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Question 10 [6 marks]

(a) [2 marks]

$h(x) = x^2 - 4x + 5 = (x-2)^2 + 1$

Vertex at $x = 2$. For $h$ to be one-one (hence invertible), need to restrict to one side of vertex.

Answer: Minimum value of $k$ is $k = 2$ [2 marks]

(b) [4 marks]

For $k = 2$: $h(x) = (x-2)^2 + 1$ for $x \geq 2$

Let $y = (x-2)^2 + 1$ with $y \geq 1$ (range of $h$)

$y - 1 = (x-2)^2$

$x - 2 = \sqrt{y-1}$ (taking positive root since $x \geq 2$)

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

Answer: $h^{-1}(x) = 2 + \sqrt{x-1}$ [2 marks]

Domain of $h^{-1}$: Range of $h$ = $[1, \infty)$ or $x \geq 1$ [1 mark]

Range of $h^{-1}$: Domain of $h$ = $[2, \infty)$ or $h^{-1}(x) \geq 2$ [1 mark]

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Question 11 [5 marks]

From first equation: $x = 5 - 2y$

Substitute into second: $(5-2y)^2 + y^2 - 2(5-2y)y = 9$ $25 - 20y + 4y^2 + y^2 - 10y + 4y^2 = 9$ $9y^2 - 30y + 25 = 9$ $9y^2 - 30y + 16 = 0$

Using formula: $y = \frac{30 \pm \sqrt{900 - 576}}{18} = \frac{30 \pm \sqrt{324}}{18} = \frac{30 \pm 18}{18}$

So $y = \frac{48}{18} = \frac{8}{3}$ or $y = \frac{12}{18} = \frac{2}{3}$

When $y = \frac{8}{3}$: $x = 5 - \frac{16}{3} = -\frac{1}{3}$

When $y = \frac{2}{3}$: $x = 5 - \frac{4}{3} = \frac{11}{3}$

Answer: $(-\frac{1}{3}, \frac{8}{3})$ and $(\frac{11}{3}, \frac{2}{3})$ [5 marks]

Marking breakdown:

• Correct substitution: [1 mark]
• Correct quadratic in y: [1 mark]
• Correct y-values: [2 marks]
• Correct corresponding x-values: [1 mark]

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Question 12 [5 marks]

(a) [2 marks]

For curve above x-axis: $x^2 - 6x + c > 0$ for all $x$.

Discriminant: $\Delta = 36 - 4c < 0$ (no real roots, and since $a = 1 > 0$, parabola opens upward)

So $c > 9$

Answer: $c > 9$ [2 marks]

(b) [3 marks]

For tangency: $x^2 - 6x + c = 2x + k$ has equal roots.

$x^2 - 8x + (c-k) = 0$

$\Delta = 64 - 4(c-k) = 0$

$64 = 4(c-k)$

$16 = c - k$

Answer: $k = c - 16$ [3 marks]

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Section B: Applications and Problem Solving

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Question 13 [6 marks]

(a) [2 marks]

Let sides perpendicular to wall be $x$ m each. Side opposite wall is $60 - 2x$ m.

Area: $A = x(60-2x) = 60x - 2x^2$ ✓ [2 marks for correct derivation with diagram interpretation]

(b) [4 marks]

Method 1: Completing the square

$A = -2x^2 + 60x = -2(x^2 - 30x)$ $= -2[(x-15)^2 - 225]$ $= -2(x-15)^2 + 450$

Maximum when $x = 15$: $A_{max} = 450$ [2 marks]

Dimensions: $x = 15$ m, other side $= 60 - 30 = 30$ m [2 marks]

Method 2: Calculus $\frac{dA}{dx} = 60 - 4x = 0$, so $x = 15$. Verify maximum: $\frac{d^2A}{dx^2} = -4 < 0$ ✓

Answer: Maximum area = 450 m²; dimensions are 15 m (perpendicular to wall) and 30 m (parallel to wall)

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Question 14 [6 marks]

(a) [1 mark]

At $t = 0$: $N = 500e^0 = 500$

Answer: 500 bacteria [1 mark]

(b) [2 marks]

$N = 500e^{0.03 \times 10} = 500e^{0.3} = 500 \times 1.34986... \approx 674.9$

Answer: 675 bacteria [2 marks]

(c) [3 marks]

$500e^{0.03t} = 2000$

$e^{0.03t} = 4$

$0.03t = \ln 4$

$t = \frac{\ln 4}{0.03} = \frac{1.386...}{0.03} \approx 46.21$

Answer: 46.21 hours [3 marks]

Marking breakdown:

• Setting correct equation: [1 mark]
• Taking logs correctly: [1 mark]
• Correct final answer to 2 d.p.: [1 mark]

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Question 15 [7 marks]

(a) [3 marks]

$f'(x) = 3x^2 - 6x - 9 = 0$

$x^2 - 2x - 3 = 0$

$(x-3)(x+1) = 0$

$x = 3$ or $x = -1$

Answer: Stationary points at $x = -1$ and $x = 3$ [3 marks]

(b) [3 marks]

$f''(x) = 6x - 6$

At $x = -1$: $f''(-1) = -12 < 0$, so local maximum [1.5 marks]

At $x = 3$: $f''(3) = 12 > 0$, so local minimum [1.5 marks]

(Alternative: first derivative test acceptable)

(c) [1 mark]

Evaluate: $f(-2) = -8 - 12 + 18 + 10 = 8$

$f(-1) = -1 - 3 + 9 + 10 = 15$ (local max)

$f(3) = 27 - 27 - 27 + 10 = -17$ (local min)

$f(5) = 125 - 75 - 45 + 10 = 15$

Answer: Range is $[-17, 15]$ [1 mark]

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Question 16 [7 marks]

(a) [4 marks]

Maximum at $(-1, 4)$, so $f(x) = a(x+1)^2 + 4$ with $a < 0$

Using $f(1) = 0$: $a(2)^2 + 4 = 0$, so $4a = -4$, $a = -1$ [2 marks]

$f(x) = -(x+1)^2 + 4 = -(x^2 + 2x + 1) + 4 = -x^2 - 2x + 3$

So $b = -2$, $c = 3$ [2 marks]

Answer: $a = -1$, $b = -2$, $c = 3$

(b) [3 marks]

From $f(x) = -(x+1)^2 + 4 = -(x+3)(x-1)$:

• Turning point (maximum): $(-1, 4)$ [1 mark]
• x-intercepts: $(-3, 0)$ and $(1, 0)$ [1 mark]
• y-intercept: $f(0) = 3$, so $(0, 3)$ [1 mark]

Expected sketch: Downward parabola with vertex $(-1,4)$, crossing x-axis at $-3$ and $1$, y-axis at $3$.

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Question 17 [7 marks]

(a) [4 marks]

$y = x + 4x^{-1}$

$\frac{dy}{dx} = 1 - \frac{4}{x^2}$ [1 mark]

For stationary points: $1 - \frac{4}{x^2} = 0$, so $x^2 = 4$, $x = \pm 2$

For $x > 0$: $x = 2$ [1 mark]

$y = 2 + \frac{4}{2} = 4$ [1 mark]

Second derivative: $\frac{d^2y}{dx^2} = \frac{8}{x^3} > 0$ when $x = 2$, so minimum. [1 mark]

Answer: Minimum value is 4

(b) [2 marks]

For two distinct intersections with $y = k$: equation $x + \frac{4}{x} = k$ has two positive solutions.

$x^2 - kx + 4 = 0$

Discriminant: $k^2 - 16 > 0$ for distinct roots, so $|k| > 4$, i.e. $k > 4$ or $k < -4$.

But for $x > 0$, minimum value is 4, and as $x \to 0^+$, $y \to +\infty$, as $x \to +\infty$, $y \to +\infty$.

So for two distinct positive solutions: need $k > 4$.

Also, if $k < 0$: one positive and one negative root (product of roots = 4 > 0 means both same sign; sum = k < 0 means both negative). So no positive solutions when $k < 0$.

Answer: $k > 4$ [2 marks]

(c) [1 mark]

At $x = 2$: $\frac{dy}{dx} = 1 - 1 = 0$. Tangent is horizontal.

Normal is vertical: $x = 2$ [1 mark]

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Question 18 [7 marks]

(a) [4 marks]

From $x^2 - 2x + 5 = 0$: $\alpha + \beta = 2$, $\alpha\beta = 5$

$\alpha^2 + \beta^2 = (\alpha+\beta)^2 - 2\alpha\beta = 4 - 10 = -6$ [2 marks]

$\alpha^3 + \beta^3 = (\alpha+\beta)^3 - 3\alpha\beta(\alpha+\beta) = 8 - 3(5)(2) = 8 - 30 = -22$ [2 marks]

Answers: $\alpha^2 + \beta^2 = -6$, $\alpha^3 + \beta^3 = -22$

(b) [3 marks]

First root: $\alpha^2 - \alpha\beta + \beta^2 = (\alpha^2+\beta^2) - \alpha\beta = -6 - 5 = -11$

Second root: $\alpha^2 + \alpha\beta + \beta^2 = -6 + 5 = -1$

Sum of new roots: $-11 + (-1) = -12$

Product: $(-11)(-1) = 11$

Answer: $x^2 + 12x + 11 = 0$ [3 marks]

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END OF ANSWER KEY