Maths Question (F.2) THX.. =]

Shing Hin

2007-08-28 7:31 pm

1. A cylindrical pen holder of base radius 4 cm, height 12.5 cm does not have a lid. Find the total external surface area of the pen holder.

2. May makes a dozen small candles melt and recast it into a large cylindrical candle of height 20 cm. If at first each small candle is a cylinder of radius 1 cm and height 9 cm, find the radius of the large candle formed.

回答 (1)

him

2007-08-28 8:41 pm

✔ 最佳答案

surface are of the base circle is πr^2
circumference of the circle is 2πr

hence external surface area of the pen holder
= πr^2 + 2πrh where h is the height of the pen holder
= π(4)^2 + 2π(4)(12.5)
=116π ......π~3.1416
~364.43 corr to 2 sig fig

So, the external surface area of the pen holder is abt 364.43 cm^2

2) firstly, we find the volume of the small candles.
= π(1)^2(9)
= 9π

hence, the volume of a dozen small candles will be 12 * 9π = 108π
so, the volume of the large candle formed will be 108π also.
let r be the radius of the large candle formed.
then,
πr^2(20) = 108π
r^2 = 5.4
r~2.32 corr to 2 sig fig

hence, the radius of the large candle is approx 2.32cm

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