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Date November 2019 Marks available 3 Reference code 19N.1.SL.TZ0.S_8
Level Standard Level Paper Paper 1 (without calculator) Time zone Time zone 0
Command term Justify Question number S_8 Adapted from N/A

Question

A small cuboid box has a rectangular base of length 3 x  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 d V d x .

[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 = 12 4 x         A1   N1

[1 mark]

a.

correct substitution into volume formula        (A1)

eg     3 x × x × y x × 3 x × ( 12 x 3 x ) ( 12 4 x ) ( x ) ( 3 x )

V = 3 x 2 ( 12 4 x ) ( = 36 x 2 12 x 3 )        A1   N2

Note: Award A0 for unfinished answers such as 3 x 2 ( 12 x 3 x ) .

[2 marks]

b.

d V d x = 72 x 36 x 2            A1A1   N2

Note: Award A1 for 72 x and A1 for 36 x 2 .

[2 marks]

c.

valid approach to find maximum           (M1)

eg       V = 0 72 x 36 x 2 = 0

correct working           (A1)

eg       x ( 72 36 x ) 72 ± 72 2 4 ( 36 ) 0 2 ( 36 ) 36 x = 72 36 x ( 2 x ) = 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 ) = 72 72 × 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 × 2 2 ( 12 4 × 2 ) ,   36 ( 2 2 ) 12 ( 2 3 )

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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