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Level 2 Certificate Further Mathematics Paper 2 (Calculator) June 2021

Legend

  • (M1) Method mark – for a correct method or step
  • (A1) Accuracy mark – for a correct value or answer
  • (B1) Independent mark – correct answer regardless of method
  • (ft) Follow through – correct calculation using previous incorrect value

Note: This paper allows the use of a calculator.

Question 1 (3 marks)

Expand and simplify \( 5(2x – 1) + 4(11 – x) \)

Give your answer in the form \( a(bx + c) \) where \( a, b \) and \( c \) are integers greater than 1.

Worked Solution

Step 1: Expand the brackets

Why: We need to multiply the terms inside the parentheses by the numbers outside to remove the brackets.

\[ 5(2x – 1) = 10x – 5 \] \[ 4(11 – x) = 44 – 4x \]

(M1) Expanding at least one bracket correctly

Step 2: Collect like terms

Why: Combine the \( x \) terms and the constant terms to simplify the expression.

\[ (10x – 4x) + (-5 + 44) \] \[ 6x + 39 \]

(M1) Correct simplified expression

Step 3: Factorise the result

Why: The question asks for the form \( a(bx + c) \) with integers \( > 1 \). We look for a common factor of 6 and 39.

Both 6 and 39 are divisible by 3.

\[ 3(2x + 13) \]

Check: \( a=3, b=2, c=13 \). All are integers greater than 1.

Final Answer:

\[ 3(2x + 13) \]

(A1)

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Question 2 (4 marks total)

Part (a) (2 marks)

\( 5m \) is decreased by 40%.

The answer is \( (m + 1) \).

Work out the value of \( m \).

Worked Solution – Part (a)

Step 1: Set up the equation

Why: Decreasing by 40% is the same as finding 60% of the value (100% – 40%).

\[ 5m \times (1 – 0.40) = m + 1 \] \[ 5m \times 0.6 = m + 1 \]

(M1) Correct method to decrease by 40%

Step 2: Solve for m
\[ 3m = m + 1 \]

Subtract \( m \) from both sides:

\[ 2m = 1 \] \[ m = 0.5 \]

Answer: \( m = 0.5 \)

(A1)

Part (b) (2 marks)

Solve \( \sqrt[3]{2w – 10} = 18 \)

Worked Solution – Part (b)

Step 1: Eliminate the cube root

Why: The inverse operation of a cube root is cubing.

\[ (\sqrt[3]{2w – 10})^3 = 18^3 \] \[ 2w – 10 = 5832 \]

(M1) Cubing 18 correctly

Step 2: Solve the linear equation
\[ 2w = 5832 + 10 \] \[ 2w = 5842 \] \[ w = \frac{5842}{2} \] \[ w = 2921 \]

Answer: \( w = 2921 \)

(A1)

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Question 3 (3 marks)

The rectangle and triangle shown have equal areas.

2d 12de 9d 8e²

Work out the value of \( \frac{d}{e} \).

Give your answer in its simplest form.

Worked Solution

Step 1: Calculate the area of the rectangle
\[ \text{Area} = \text{width} \times \text{height} \] \[ \text{Area} = 12de \times 2d = 24d^2e \]
Step 2: Calculate the area of the triangle
\[ \text{Area} = \frac{1}{2} \times \text{base} \times \text{height} \] \[ \text{Area} = \frac{1}{2} \times 8e^2 \times 9d = 36de^2 \]

(M1) Expressions for both areas

Step 3: Equate areas and simplify

Why: The problem states the areas are equal.

\[ 24d^2e = 36de^2 \]

Divide both sides by \( 12de \) (assuming \( d, e \neq 0 \)):

\[ \frac{24d^2e}{12de} = \frac{36de^2}{12de} \] \[ 2d = 3e \]

(M1) Simplifying the equation

Step 4: Rearrange to find d/e

Divide both sides by \( e \):

\[ \frac{2d}{e} = 3 \]

Divide by 2:

\[ \frac{d}{e} = \frac{3}{2} \]

Answer: \( 1.5 \) or \( \frac{3}{2} \)

(A1)

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Question 4 (3 marks)

The equations of the two circles shown are

\( x^2 + y^2 = 100 \) and \( x^2 + y^2 = 36 \)

x y O

Work out the shaded area.

Give your answer as an integer multiple of \( \pi \).

Worked Solution

Step 1: Identify the radii

Why: The equation of a circle is \( x^2 + y^2 = r^2 \).

Outer circle: \( r^2 = 100 \Rightarrow r = 10 \)

Inner circle: \( r^2 = 36 \Rightarrow r = 6 \)

Step 2: Calculate the area of the full annulus

Why: The area between two circles is Area(outer) – Area(inner).

\[ \text{Area}_{\text{outer}} = \pi \times 10^2 = 100\pi \] \[ \text{Area}_{\text{inner}} = \pi \times 6^2 = 36\pi \] \[ \text{Full Annulus} = 100\pi – 36\pi = 64\pi \]

(M1) Calculating full area difference

Step 3: Calculate shaded fraction

Why: The diagram shows the shaded region is in one quadrant (the top-right), which is \( \frac{1}{4} \) of the total circle.

\[ \text{Shaded Area} = \frac{1}{4} \times 64\pi \] \[ \text{Shaded Area} = 16\pi \]

(M1) Using the quarter fraction

Answer: \( 16\pi \) units²

(A1)

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Question 5 (3 marks)

\( SQR \) is a right-angled triangle.

\( P \) is a point on \( SQ \).

Angle \( SPR = 45^\circ \).

\( M \) is the midpoint of \( QR \).

\( k \) is a constant.

O x y S R(k, 15) Q(1, 1) P(1, 6) M 45°

Work out the coordinates of \( M \).

Worked Solution

Step 1: Determine the coordinates of S

Why: \( SQR \) is a right-angled triangle. Looking at the diagram, \( SQ \) is vertical (same x-coordinate) and \( SR \) is horizontal (same y-coordinate).

\( Q \) is at \( (1, 1) \) and \( P \) is at \( (1, 6) \). Since \( S \) is on the line extending \( QP \) upwards, \( S \) has x-coordinate 1.

\( R \) is at \( (k, 15) \). Since \( SR \) is horizontal, \( S \) has y-coordinate 15.

So, \( S = (1, 15) \).

Step 2: Use the angle SPR to find k

Why: Consider the triangle \( PSR \). \( S \) is a right angle (intersection of vertical and horizontal lines).

Length \( PS = \text{y-coord of S} – \text{y-coord of P} = 15 – 6 = 9 \).

Length \( SR = \text{x-coord of R} – \text{x-coord of S} = k – 1 \).

In triangle \( PSR \), angle \( SPR = 45^\circ \).

\[ \tan(45^\circ) = \frac{\text{Opposite}}{\text{Adjacent}} = \frac{SR}{PS} \] \[ 1 = \frac{k – 1}{9} \] \[ 9 = k – 1 \Rightarrow k = 10 \]

So \( R \) is \( (10, 15) \).

(M1) Finding coordinates of R

Step 3: Calculate midpoint M

Why: \( M \) is the midpoint of \( QR \).

\( Q = (1, 1) \)

\( R = (10, 15) \)

\[ M = \left( \frac{1 + 10}{2}, \frac{1 + 15}{2} \right) \] \[ M = \left( \frac{11}{2}, \frac{16}{2} \right) \] \[ M = (5.5, 8) \]

(M1) Midpoint formula

Answer: \( (5.5, 8) \)

(A1)

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Question 6 (3 marks)

Rearrange \( y = \sqrt{\frac{x + 2w}{3}} \) to make \( w \) the subject.

Worked Solution

Step 1: Square both sides

Why: We need to remove the square root to access the term with \( w \).

\[ y^2 = \frac{x + 2w}{3} \]

(M1)

Step 2: Multiply by 3
\[ 3y^2 = x + 2w \]

(M1)

Step 3: Isolate w

Subtract \( x \) from both sides:

\[ 3y^2 – x = 2w \]

Divide by 2:

\[ w = \frac{3y^2 – x}{2} \]

Answer: \( w = \frac{3y^2 – x}{2} \)

(A1)

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Question 7 (4 marks total)

Part (a) (2 marks)

\( a \) is a value greater than 1.

Work out the value of \( m \) for which \( (a^m)^4 = (a^5)^{2m} \)

Worked Solution – Part (a)

Step 1: Simplify the indices

Why: Use the power law \( (x^a)^b = x^{ab} \).

\[ a^{4m} = a^{10m} \]

(M1)

Step 2: Equate exponents
\[ 4m = 10m \] \[ 6m = 0 \] \[ m = 0 \]

Answer: \( m = 0 \)

(A1)

Part (b) (2 marks)

\( w^3 x^2 y^5 = w^{13} x^7 \)

Write \( y \) in terms of \( w \) and \( x \).

Give your answer in its simplest form.

Worked Solution – Part (b)

Step 1: Isolate y⁵
\[ y^5 = \frac{w^{13} x^7}{w^3 x^2} \] \[ y^5 = w^{13-3} x^{7-2} \] \[ y^5 = w^{10} x^5 \]

(M1) Correct division of indices

Step 2: Find the 5th root
\[ y = \sqrt[5]{w^{10} x^5} \] \[ y = (w^{10})^{1/5} (x^5)^{1/5} \] \[ y = w^2 x \]

Answer: \( y = w^2 x \)

(A1)

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Question 8 (4 marks)

A function \( f \) is given by

\( f(x) = 4x \) for \( x < 0 \)

\( f(x) = x^2 – 8x \) for \( 0 \leqslant x \leqslant 8 \)

\( f(x) = 16 – 2x \) for \( x > 8 \)

A sketch of \( y = f(x) \) is shown.

x y O (4, -16) 8

Work out all the values of \( x \) for which \( f(x) = -12 \).

Worked Solution

Step 1: Check the first part of the function (\( x < 0 \))
\[ 4x = -12 \] \[ x = -3 \]

Is \( x < 0 \)? Yes. So \( x = -3 \) is a solution.

(B1) First value

Step 2: Check the middle part (\( 0 \leqslant x \leqslant 8 \))
\[ x^2 – 8x = -12 \] \[ x^2 – 8x + 12 = 0 \]

Factorise:

\[ (x – 2)(x – 6) = 0 \] \[ x = 2 \text{ or } x = 6 \]

Are these in range \( [0, 8] \)? Yes. So \( x = 2 \) and \( x = 6 \) are solutions.

(B2) Correct quadratic solutions

Step 3: Check the third part (\( x > 8 \))
\[ 16 – 2x = -12 \] \[ -2x = -28 \] \[ x = 14 \]

Is \( x > 8 \)? Yes. So \( x = 14 \) is a solution.

(B1) Fourth value

Answer: \( -3, 2, 6, 14 \)

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Question 9 (4 marks total)

Part (a) (1 mark)

Circle the expression that is equivalent to \( \frac{1}{a} + \frac{1}{b} \)

\( \frac{2}{a+b} \quad \frac{ab}{b+a} \quad \frac{2}{ab} \quad \frac{b+a}{ab} \)

Worked Solution – Part (a)

Find a common denominator (\( ab \)):

\[ \frac{1}{a} + \frac{1}{b} = \frac{b}{ab} + \frac{a}{ab} = \frac{b+a}{ab} \]

Answer: \( \frac{b+a}{ab} \)

(B1)

Part (b) (3 marks)

Simplify fully \( \frac{6c^4 – c^3}{36c^2 – 1} \)

Worked Solution – Part (b)

Step 1: Factorise the numerator

Take out common factor \( c^3 \):

\[ 6c^4 – c^3 = c^3(6c – 1) \]
Step 2: Factorise the denominator

Why: This is a difference of two squares \( (A^2 – B^2) = (A-B)(A+B) \).

\[ 36c^2 – 1 = (6c)^2 – 1^2 = (6c – 1)(6c + 1) \]

(B1) Correct factorisation

Step 3: Simplify
\[ \frac{c^3(6c – 1)}{(6c – 1)(6c + 1)} \]

Cancel the \( (6c – 1) \) terms:

\[ \frac{c^3}{6c + 1} \]

Answer: \( \frac{c^3}{6c + 1} \)

(B2)

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Question 10 (3 marks)

The radius of a sphere, in cm, is \( \frac{3k}{2} \)

The volume of the sphere, in cm³, is \( 972\pi \)

Work out the value of \( k \).

(Volume of a sphere = \( \frac{4}{3}\pi r^3 \))

Worked Solution

Step 1: Substitute values into the formula
\[ \text{Volume} = \frac{4}{3}\pi r^3 \] \[ 972\pi = \frac{4}{3}\pi \left( \frac{3k}{2} \right)^3 \]

(M1) Setting up equation

Step 2: Simplify and solve

Divide by \( \pi \):

\[ 972 = \frac{4}{3} \left( \frac{27k^3}{8} \right) \] \[ 972 = \frac{1}{3} \left( \frac{27k^3}{2} \right) \] \[ 972 = \frac{9k^3}{2} \] \[ 972 = 4.5 k^3 \]

(M1) Rearranging for k³

Step 3: Final calculation
\[ k^3 = \frac{972}{4.5} = 216 \] \[ k = \sqrt[3]{216} \] \[ k = 6 \]

Answer: \( k = 6 \)

(A1)

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Question 11 (3 marks)

Expand and simplify fully \( (5x + 3y^2)(4x – y^2) \)

Worked Solution

Step 1: Multiply terms (FOIL method)
\[ 5x \times 4x = 20x^2 \] \[ 5x \times (-y^2) = -5xy^2 \] \[ 3y^2 \times 4x = 12xy^2 \] \[ 3y^2 \times (-y^2) = -3y^4 \]

(M1) 3 out of 4 terms correct

Step 2: Collect like terms

Why: The terms \( -5xy^2 \) and \( 12xy^2 \) are like terms.

\[ 20x^2 + (-5 + 12)xy^2 – 3y^4 \] \[ 20x^2 + 7xy^2 – 3y^4 \]

(A1) Correct simplified expression

Answer: \( 20x^2 + 7xy^2 – 3y^4 \)

(A1)

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Question 12 (5 marks)

\( A \) and \( B \) are points on the line \( y = 3x + 2 \)

\( B \), \( C \) and \( D(5, 0) \) are points on the line \( L \).

\( OA : AC = 1 : 4 \)

x y O y = 3x + 2 line L A C B D

Work out the x-coordinate of \( B \).

Worked Solution

Step 1: Find coordinates of A and C

Why: \( A \) is on the line \( y=3x+2 \) and on the y-axis (from diagram). \( C \) is on the y-axis.

For \( A \), let \( x=0 \) in \( y=3x+2 \): \( y=2 \). So \( A \) is \( (0, 2) \).

\( OA \) represents the distance from origin to \( A \), which is 2 units.

Ratio \( OA : AC = 1 : 4 \).

\[ \frac{2}{AC} = \frac{1}{4} \Rightarrow AC = 8 \]

\( C \) is 8 units above \( A \). \( y_C = 2 + 8 = 10 \).

So \( C \) is \( (0, 10) \).

(B1) Coordinates of C

Step 2: Find the equation of Line L

Why: Line L passes through \( C(0, 10) \) and \( D(5, 0) \).

Gradient \( m = \frac{y_2 – y_1}{x_2 – x_1} = \frac{0 – 10}{5 – 0} = \frac{-10}{5} = -2 \).

Equation: \( y = -2x + c \). Since y-intercept is 10:

\[ y = -2x + 10 \]

(M1) Equation of line L

Step 3: Find intersection point B

Why: \( B \) is where the two lines cross. Set their equations equal.

\[ 3x + 2 = -2x + 10 \] \[ 5x = 8 \] \[ x = \frac{8}{5} = 1.6 \]

(A1) Correct x-coordinate

Answer: \( 1.6 \)

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Question 13 (4 marks)

\( P \) is the point on the curve \( y = ax^3 + 10x^2 \) where \( x = 2 \).

The gradient of the normal to the curve at \( P \) is \( -\frac{1}{4} \).

Work out the value of \( a \).

Worked Solution

Step 1: Find gradient of the tangent

Why: The normal and tangent are perpendicular. \( m_{\text{normal}} \times m_{\text{tangent}} = -1 \).

\[ -\frac{1}{4} \times m_{\text{tangent}} = -1 \] \[ m_{\text{tangent}} = 4 \]

(M1)

Step 2: Differentiate the curve equation
\[ \frac{dy}{dx} = 3ax^2 + 20x \]

(M1) Differentiation

Step 3: Equate gradients at x = 2

Substitute \( x = 2 \) into derivative:

\[ 3a(2)^2 + 20(2) = 12a + 40 \]

Set equal to tangent gradient 4:

\[ 12a + 40 = 4 \] \[ 12a = -36 \] \[ a = -3 \]

Answer: \( a = -3 \)

(A1)

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Question 14 (3 marks total)

Part (a) (1 mark)

\( \mathbf{A} = \begin{pmatrix} 1 & 0 \\ 0 & -1 \end{pmatrix} \)

Describe geometrically the single transformation represented by \( \mathbf{A} \).

Worked Solution – Part (a)

Applying to unit vectors:

\( \begin{pmatrix} 1 \\ 0 \end{pmatrix} \to \begin{pmatrix} 1 \\ 0 \end{pmatrix} \) (x stays same)

\( \begin{pmatrix} 0 \\ 1 \end{pmatrix} \to \begin{pmatrix} 0 \\ -1 \end{pmatrix} \) (y becomes negative)

This is a Reflection in the x-axis.

(B1)

Part (b) (2 marks)

\( \mathbf{B} = \begin{pmatrix} 0 & 1 \\ -1 & 0 \end{pmatrix} \)

Describe geometrically the single transformation represented by \( \mathbf{B}^2 \).

Worked Solution – Part (b)

Step 1: Calculate B²
\[ \mathbf{B}^2 = \begin{pmatrix} 0 & 1 \\ -1 & 0 \end{pmatrix} \begin{pmatrix} 0 & 1 \\ -1 & 0 \end{pmatrix} \] \[ = \begin{pmatrix} (0)(0)+(1)(-1) & (0)(1)+(1)(0) \\ (-1)(0)+(0)(-1) & (-1)(1)+(0)(0) \end{pmatrix} \] \[ = \begin{pmatrix} -1 & 0 \\ 0 & -1 \end{pmatrix} \]

(M1) Matrix multiplication

Step 2: Identify transformation

\( x \to -x \) and \( y \to -y \).

This is a Rotation of 180° about the origin (or Enlargement scale factor -1).

Answer: Rotation 180° about the origin

(B1)

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Question 15 (5 marks)

\( A \), \( B \) and \( C \) are points on a circle, centre \( O \).

\( ACD \) is a straight line.

Angle \( BCD = w \).

O D A C B x w

Prove that \( w = x + 90^\circ \).

Worked Solution

Step 1: Analyze triangle OAB

Why: \( OA \) and \( OB \) are radii, so triangle \( OAB \) is isosceles.

\( \angle OBA = \angle OAB = x \) (base angles of isosceles triangle).

Sum of angles in triangle \( OAB \):

\[ \angle AOB = 180 – (x + x) = 180 – 2x \]

(M1) Finding angle AOB

Step 2: Find angle ACB

Why: The angle at the centre is double the angle at the circumference.

\[ \angle ACB = \frac{1}{2} \times \angle AOB \] \[ \angle ACB = \frac{1}{2} (180 – 2x) = 90 – x \]

(M1) Circle theorem application

Step 3: Use the straight line property

Why: Angles on a straight line add to 180°.

\[ \angle ACB + \angle BCD = 180^\circ \] \[ (90 – x) + w = 180 \]

(M1) Straight line equation

Step 4: Rearrange to prove
\[ w = 180 – (90 – x) \] \[ w = 180 – 90 + x \] \[ w = 90 + x \]

QED

(A1) Complete proof

Proven: \( w = x + 90^\circ \)

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Question 16 (3 marks)

The coefficient of \( x^4 \) in the expansion of \( (a + 2x)^6 \) is 1500.

Work out the two possible values of \( a \).

Worked Solution

Step 1: Find the general term for x⁴

Why: In binomial expansion \( (A+B)^n \), the term is \( \binom{n}{r} A^{n-r} B^r \). Here \( n=6 \), we want \( x^4 \) so we look at \( (2x)^4 \).

Term involves \( \binom{6}{4} a^2 (2x)^4 \).

\[ \binom{6}{4} = 15 \] \[ (2x)^4 = 16x^4 \]

Term is: \( 15 \times a^2 \times 16x^4 = 240a^2 x^4 \)

(M1) Establishing coefficient expression

Step 2: Solve for a
\[ 240a^2 = 1500 \] \[ a^2 = \frac{1500}{240} = \frac{150}{24} = 6.25 \]

(M1) Equation for a²

Step 3: Final values
\[ a = \pm \sqrt{6.25} \] \[ a = \pm 2.5 \]

Answer: \( 2.5 \) and \( -2.5 \)

(A1)

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Question 17 (5 marks)

\( ABCDEFGH \) is a cube with side length 32 cm.

\( M \) and \( N \) are points on \( DH \) and \( CG \) respectively.

A B C D E F G H M N 28°

The angle marked at \( M \) (between \( MN \) and \( MG \)) is \( 28^\circ \).

Work out the size of the angle that the line \( BM \) makes with the plane \( ABCD \).

Worked Solution

Step 1: Calculate the height of M from the top face

Why: In triangle \( MNG \), \( N \) is at the same height as \( M \) (horizontal line). \( G \) is the corner above \( N \).

\( MN = 32 \) (width of cube).

\( \tan(28^\circ) = \frac{\text{Opp}}{\text{Adj}} = \frac{GN}{MN} \).

\[ GN = 32 \times \tan(28^\circ) \]
Step 2: Calculate height MD

Why: \( M \) is on \( DH \). \( H \) is at top, \( D \) is at bottom. \( D, C, G, H \) is a rectangle. \( NC = MD \).

Height of cube \( CG = 32 \).

\( NC = CG – GN = 32 – 32 \tan(28^\circ) \).

\( MD = NC = 32(1 – \tan 28^\circ) \).

Step 3: Calculate the diagonal BD

Why: We need the base of the right-angled triangle \( MDB \) to find the angle at \( B \).

Base \( ABCD \) is a square of side 32.

\[ BD = \sqrt{32^2 + 32^2} = 32\sqrt{2} \]
Step 4: Calculate the required angle

Let the angle be \( \theta \). In right-angled triangle \( MDB \):

\[ \tan \theta = \frac{MD}{BD} \] \[ \tan \theta = \frac{32(1 – \tan 28^\circ)}{32\sqrt{2}} \] \[ \tan \theta = \frac{1 – \tan 28^\circ}{\sqrt{2}} \] \[ \tan \theta \approx \frac{1 – 0.5317}{1.414} \] \[ \tan \theta \approx \frac{0.4683}{1.414} \approx 0.331 \] \[ \theta = \tan^{-1}(0.331…) \] \[ \theta \approx 18.3^\circ \]

Answer: \( 18.3^\circ \)

(A1) [Accept 18.3 to 18.4]

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Question 18 (5 marks)

\( y = 12x + \frac{3}{x} \)

Show that \( y \) has a minimum value when \( x = 0.5 \).

Worked Solution

Step 1: Differentiate y

Why: To find stationary points (max/min), we need the derivative \( \frac{dy}{dx} \).

Rewrite \( \frac{3}{x} \) as \( 3x^{-1} \).

\[ y = 12x + 3x^{-1} \] \[ \frac{dy}{dx} = 12 – 3x^{-2} = 12 – \frac{3}{x^2} \]

(M1) Correct differentiation

Step 2: Show stationary point at x = 0.5

Set \( \frac{dy}{dx} = 0 \) and substitute \( x = 0.5 \):

\[ 12 – \frac{3}{(0.5)^2} = 12 – \frac{3}{0.25} \] \[ = 12 – 12 = 0 \]

So \( x = 0.5 \) is a stationary point.

(M1) Verifying derivative is zero

Step 3: Determine nature of the point

Why: Use the second derivative \( \frac{d^2y}{dx^2} \). Positive = Minimum.

\[ \frac{d^2y}{dx^2} = -3(-2)x^{-3} = 6x^{-3} = \frac{6}{x^3} \]

At \( x = 0.5 \):

\[ \frac{6}{(0.5)^3} = \frac{6}{0.125} = 48 \]

Since \( 48 > 0 \), it is a minimum.

(A1) Proof complete

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Question 19 (3 marks total)

Part (a) (1 mark)

\( f(x) = (x + 2)^3 \)

\( g \) is a function such that \( gf(x) = (x + 2)^{12} \)

Work out an expression for \( g(x) \).

Worked Solution – Part (a)

We know \( f(x) = (x+2)^3 \).

We want \( g(f(x)) = ((x+2)^3)^4 = (x+2)^{12} \).

So \( g \) raises the input to the power of 4.

\[ g(x) = x^4 \]

Answer: \( g(x) = x^4 \)

Part (b) (2 marks)

\( h(x) = x^2 + 5 \)

\( k \) is a function such that \( hk(x) = 4x^2 + 5 \)

Work out an expression for \( kh(x) \).

Worked Solution – Part (b)

Step 1: Find k(x)

\( h(k(x)) = (k(x))^2 + 5 \).

We are given \( hk(x) = 4x^2 + 5 \).

\[ (k(x))^2 + 5 = 4x^2 + 5 \] \[ (k(x))^2 = 4x^2 \] \[ k(x) = 2x \] (assuming positive for standard function notation)

(M1)

Step 2: Find kh(x)
\[ k(h(x)) = 2(h(x)) \] \[ = 2(x^2 + 5) \] \[ = 2x^2 + 10 \]

Answer: \( 2x^2 + 10 \)

(A1)

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Question 20 (4 marks)

Show that \( \frac{2\sin x + \cos x}{\tan x} – \frac{1}{\sin x} \) can be written in the form \( a\cos x + b\sin x \)

where \( a \) and \( b \) are integers.

Worked Solution

Step 1: Substitute tan x

Use \( \tan x = \frac{\sin x}{\cos x} \):

\[ \frac{2\sin x + \cos x}{\frac{\sin x}{\cos x}} – \frac{1}{\sin x} \] \[ = \frac{\cos x(2\sin x + \cos x)}{\sin x} – \frac{1}{\sin x} \]

(M1)

Step 2: Combine fractions
\[ \frac{2\sin x \cos x + \cos^2 x – 1}{\sin x} \]

(M1) Common denominator

Step 3: Use Identity

Use \( \sin^2 x + \cos^2 x = 1 \Rightarrow \cos^2 x – 1 = -\sin^2 x \):

\[ \frac{2\sin x \cos x – \sin^2 x}{\sin x} \]

(M1) Pythagorean identity

Step 4: Simplify

Divide each term by \( \sin x \):

\[ \frac{2\sin x \cos x}{\sin x} – \frac{\sin^2 x}{\sin x} \] \[ = 2\cos x – \sin x \]

Answer: \( 2\cos x – 1\sin x \) (so \( a=2, b=-1 \))

(A1)

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Question 21 (6 marks)

\( 3x^2 + 2bx + 8a \) can be written in the form \( 3(x + a)^2 + b + 2 \)

Work out the two possible pairs of values of \( a \) and \( b \).

Worked Solution

Step 1: Expand the completed square form
\[ 3(x + a)^2 + b + 2 = 3(x^2 + 2ax + a^2) + b + 2 \] \[ = 3x^2 + 6ax + 3a^2 + b + 2 \]

(M1) Expansion

Step 2: Equate coefficients

Why: Since the expressions are identical, coefficients of \( x \) must match and constants must match.

Coefficient of \( x \): \( 2b = 6a \Rightarrow b = 3a \)

Constant term: \( 8a = 3a^2 + b + 2 \)

(M1) Setting up simultaneous equations

Step 3: Solve the system

Substitute \( b = 3a \) into the second equation:

\[ 8a = 3a^2 + (3a) + 2 \] \[ 3a^2 – 5a + 2 = 0 \]

(M1) Quadratic in ‘a’

Factorise:

\[ (3a – 2)(a – 1) = 0 \] \[ a = \frac{2}{3} \text{ or } a = 1 \]
Step 4: Find corresponding b values

If \( a = 1 \), \( b = 3(1) = 3 \).

If \( a = \frac{2}{3} \), \( b = 3(\frac{2}{3}) = 2 \).

Answers:

\( a = 1, b = 3 \)

\( a = \frac{2}{3}, b = 2 \)

(A1) Both pairs correct

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