Calculus - Rate of change

• Han chiang

  10 Dec 10, 14:59

  Hi there, I have encountered a very unusual problem with rate of change. This is the question:

  A man 1.5 m tall walks at the rate of 1.2 m/s directly away from a street light which is 6m above the street.

  i) At what rate is the tip of his shadow changing?

  ii) At what rate is the length of his shadow changing?

  Ans: 1.6 m/s and 0.4 m/s

  After my friend explained to me, I still don't quite get it. And how do I draw a diagram for this? I am really bad at diagrams...

• qdtimes2

  10 Dec 10, 23:32

  not sure...... this is my diagram..

  

  i) find dx/dB, then find dx/dt

  ii) find  dy/dt

   

   

• Han chiang

  12 Dec 10, 21:07

  Ok I get it now, its done using similar triangles and just using some letters to represent the lengths. Thanks alot man!

• eagle

  17 Dec 10, 08:43

  Someone asked a similar question in march 2009 last year in sgforums.

  I have the question on my website:

  http://examworld.blogspot.com/2009/03/o-lvl-maths-applications-of.html

  Think is one of the textbook questions