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Date November 2019 Marks available 2 Reference code 19N.1.SL.TZ0.S_8
Level Standard Level Paper Paper 1 (without calculator) Time zone Time zone 0
Command term Find Question number S_8 Adapted from N/A

Question

A small cuboid box has a rectangular base of length 3x cm and width x cm, where x>0. The height is y cm, where y>0.

The sum of the length, width and height is 12 cm.

The volume of the box is V cm3.

Write down an expression for y in terms of x.

[1]
a.

Find an expression for V in terms of x.

[2]
b.

Find dVdx.

[2]
c.

Find the value of x for which V is a maximum.

[4]
d.i.

Justify your answer.

[3]
d.ii.

Find the maximum volume.

[2]
e.

Markscheme

y=124x        A1   N1

[1 mark]

a.

correct substitution into volume formula        (A1)

eg     3x×x×yx×3x×(12x3x)(124x)(x)(3x)

V=3x2(124x)(=36x212x3)       A1   N2

Note: Award A0 for unfinished answers such as 3x2(12x3x).

[2 marks]

b.

dVdx=72x36x2           A1A1   N2

Note: Award A1 for 72x and A1 for 36x2.

[2 marks]

c.

valid approach to find maximum           (M1)

eg      V=072x36x2=0

correct working           (A1)

eg      x(7236x)72±7224(36)02(36)36x=7236x(2x)=0

x=2           A2   N2

Note: Award A1 for x=2 and x=0.

[4 marks]

d.i.

valid approach to explain that V is maximum when x=2         (M1)

eg      attempt to find V, sign chart (must be labelled V)

correct value/s         A1

eg      V(2)=7272×2,  V(a)  where  a<2  and  V(b) where  b>2

correct reasoning         R1

eg      V(2)<0,  V  is positive for  x<2  and negative for  x>2

Note: Do not award R1 unless A1 has been awarded.

V is maximum when x=2           AG   N0

[3 marks]

d.ii.

correct substitution into their expression for volume        A1

eg      3×22(124×2),  36(22)12(23)

V=48 (cm3)           A1   N1

[2 marks]

e.

Examiners report

[N/A]
a.
[N/A]
b.
[N/A]
c.
[N/A]
d.i.
[N/A]
d.ii.
[N/A]
e.

Syllabus sections

Topic 5 —Calculus » SL 5.5—Integration introduction, areas between curve and x axis
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