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Date November 2008 Marks available 8 Reference code 08N.1.hl.TZ0.9
Level HL only Paper 1 Time zone TZ0
Command term Justify and Show that Question number 9 Adapted from N/A

Question

A packaging company makes boxes for chocolates. An example of a box is shown below. This box is closed and the top and bottom of the box are identical regular hexagons of side x cm.

 

 

(a)     Show that the area of each hexagon is 33x22cm2 .

(b)     Given that the volume of the box is 90 cm2 , show that when x=320 the total surface area of the box is a minimum, justifying that this value gives a minimum.

Markscheme

(a)     Area of hexagon =6×12×x×x×sin60     M1

=33x22     AG

 

(b)     Let the height of the box be h

Volume =33hx22=90     M1

Hence h=603x2     A1

Surface area, A=33x2+6hx     M1

=33x2+3603x1     A1

dAdx=63x3603x2     A1

(dAdx=0)

63x3=3603     M1

x3=20

x=320     AG

d2Adx2=63+720x33

which is positive when x=320, and hence gives a minimum value.     R1

[8 marks]

Examiners report

There were a number of wholly correct answers seen and the best candidates tackled the question well. However, many candidates did not seem to understand what was expected in such a problem. It was disappointing that a significant number of candidates were unable to find the area of the hexagon.

Syllabus sections

Topic 6 - Core: Calculus » 6.3 » Optimization problems.

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