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 3√3x22cm2 .
(b) Given that the volume of the box is 90 cm2 , show that when x=3√20 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
=3√3x22 AG
(b) Let the height of the box be h
Volume =3√3hx22=90 M1
Hence h=60√3x2 A1
Surface area, A=3√3x2+6hx M1
=3√3x2+360√3x−1 A1
dAdx=6√3x−360√3x−2 A1
(dAdx=0)
6√3x3=360√3 M1
x3=20
x=3√20 AG
d2Adx2=6√3+720x−3√3
which is positive when x=3√20, 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.