# IB DP Maths Topic 2.4 The form x↦a(x−p)(x−q) , x-intercepts (p,0) and (q,0) SL Paper 2

 Marks available 3 Reference code 09N.2.sl.TZ0.9

## Question

Let $$f(x) = 5\cos \frac{\pi }{4}x$$ and $$g(x) = – 0.5{x^2} + 5x – 8$$ for $$0 \le x \le 9$$ .

On the same diagram, sketch the graphs of f and g .

[3]
a.

Consider the graph of $$f$$ . Write down

(i)     the x-intercept that lies between $$x = 0$$ and $$x = 3$$ ;

(ii)    the period;

(iii)   the amplitude.

[4]
b.

Consider the graph of g . Write down

(i)     the two x-intercepts;

(ii)    the equation of the axis of symmetry.

[3]
c.

Let R be the region enclosed by the graphs of f and g . Find the area of R.

[5]
d.

## Markscheme

A1A1A1     N3

Note: Award A1 for f being of sinusoidal shape, with 2 maxima and one minimum, A1 for g being a parabola opening down, A1 for two intersection points in approximately correct position.

[3 marks]

a.

(i)  $$(2{\text{, }}0)$$ (accept $$x = 2$$ )     A1     N1

(ii) $${\text{period}} = 8$$     A2     N2

(iii) $${\text{amplitude}} = 5$$     A1     N1

[4 marks]

b.

(i) $$(2{\text{, }}0)$$ , $$(8{\text{, }}0)$$ (accept $$x = 2$$ , $$x = 8$$ )     A1A1     N1N1

(ii) $$x = 5$$ (must be an equation)     A1     N1

[3 marks]

c.

METHOD 1

intersect when $$x = 2$$ and $$x = 6.79$$ (may be seen as limits of integration)     A1A1

evidence of approach     (M1)

e.g. $$\int {g – f}$$ , $$\int {f(x){\rm{d}}x – \int {g(x){\rm{d}}x}}$$ , $$\int_2^{6.79} {\left( {( – 0.5{x^2} + 5x – 8) – \left( {5\cos \frac{\pi }{4}x} \right)} \right)}$$

$${\text{area}} = 27.6$$     A2     N3

METHOD 2

intersect when $$x = 2$$ and $$x = 6.79$$ (seen anywhere)     A1A1

evidence of approach using a sketch of g and f , or $$g – f$$ .     (M1)

e.g. area = $$A + B – C$$ , $$12.7324 + 16.0938 – 1.18129 \ldots$$

$${\text{area}} = 27.6$$     A2     N3

[5 marks]

d.

## Examiners report

Graph sketches were much improved over previous sessions. Most candidates graphed the two functions correctly, but many ignored the domain restrictions.

a.

Many candidates found parts (b) and (c) accessible, although quite a few did not know how to find the period of the cosine function.

b.

Many candidates found parts (b) and (c) accessible, although quite a few did not know how to find the period of the cosine function.

c.

Part (d) proved elusive to many candidates. Some used creative approaches that split the area into parts above and below the x-axis; while this leads to a correct result, few were able to achieve it. Many candidates were unable to use their GDCs effectively to find points of intersection and the subsequent area.

d.
 Marks available 3 Reference code 13N.2.sl.TZ0.2

## Question

Let $$f(x) = (x – 1)(x – 4)$$.

Find the $$x$$-intercepts of the graph of $$f$$.

[3]
a.

The region enclosed by the graph of $$f$$ and the $$x$$-axis is rotated $$360^\circ$$ about the $$x$$-axis.

Find the volume of the solid formed.

[3]
b.

## Markscheme

valid approach     (M1)

eg     $$f(x) = 0$$, sketch of parabola showing two $$x$$-intercepts

$$x = 1,{\text{ }}x = 4{\text{ }}\left( {{\text{accept (1, 0), (4, 0)}}} \right)$$     A1A1     N3

[3 marks]

a.

attempt to substitute either limits or the function into formula involving $${f^2}$$     (M1)

eg     $$\int_1^4 {{{\left( {f(x)} \right)}^2}{\text{d}}x,{\text{ }}\pi \int {{{\left( {(x – 1)(x – 4)} \right)}^2}} }$$

$${\text{volume}} = 8.1\pi {\text{ (exact), 25.4}}$$     A2     N3

[3 marks]

b.

## Examiners report

[N/A]
a.
[N/A]
b.
 Marks available 3 Reference code 14M.2.sl.TZ2.2

## Question

Let $$f(x) = 5 – {x^2}$$. Part of the graph of $$f$$is shown in the following diagram.

The graph crosses the $$x$$-axis at the points $$\rm{A}$$ and $$\rm{B}$$.

Find the $$x$$-coordinate of $${\text{A}}$$ and of $${\text{B}}$$.

[3]
a.

The region enclosed by the graph of $$f$$ and the $$x$$-axis is revolved $$360^\circ$$ about the $$x$$-axis.

Find the volume of the solid formed.

[3]
b.

## Markscheme

recognizing $$f(x) = 0$$     (M1)

eg     $$f = 0,{\text{ }}{x^2} = 5$$

$$x = \pm 2.23606$$

$$x = \pm \sqrt 5 {\text{ (exact), }}x = \pm 2.24$$     A1A1     N3

[3 marks]

a.

attempt to substitute either limits or the function into formula

involving $${f^2}$$     (M1)

eg     $$\pi \int {{{\left( {5 – {x^2}} \right)}^2}{\text{d}}x,{\text{ }}\pi \int_{ – 2.24}^{2.24} {\left( {{x^4} – 10{x^2} + 25} \right)} ,{\text{ }}2\pi \int_0^{\sqrt 5 } {{f^2}} }$$

$$187.328$$

volume $$= 187$$     A2     N3

[3 marks]

b.

[N/A]
a.
[N/A]
b.
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