MATHS MC~~

Let the distance between A and B be d
The time he goes from A to B= d/9
The tme he goes from B to A =d/6

Total time of travel = d/9+d/6=5d/18

Total distance of travel = 2d

Average speed=Total distance/total time
=2d/(5d/18)
=7.2m/s



p.s 其實遇到呢類行呢行去然後計平均速度既問題，仲有一個聰明d既方法計:

呢個方法就係用Harmonic mean,中文係調和平均數:

Harmonic mean=n/(1/a1+1/a2+1/a3+...1/an)

比如有個人由以速S1行去B,以速S2行番去A,再以速S3行去B,佢平均速就係:

Average speed=3/(1/S1+1/S2+1/S3)

睇番您嗰題

average speed=2/(1/9+1/6)=7.2m/s

無論個人來回AB點幾多次呢嗰方法都work架構

D. 7.2

2007-04-22 00:15:01 補充：
假設AB距離18m，路途共長36m，去程用2s，回程用3s！平均速度係 36/(2 3) = 7.2m/s