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 #2
avatar+11912 
+5
Jul 20, 2014
 #1
avatar+118723 
+8

EDIT:  This answer is correct just rediculously long.  I have redone it much more efficiently on a later post.

The two triangles are similar because they each have a right angle and they have a common angle.

Let the third angle be  $$\theta$$

You know that      $$\frac{dx}{dt}=1m/s$$

a) you are being asked to find  $$\frac{dy}{dt}$$  when x=6m

b) you are being asked to find   $$\frac{d(y-x)}{dt}=\frac{dy}{dt}-\frac{dx}{dt}$$

 

a)  I think it will be easier for me to get y and x in terms of $$\theta$$

$$\begin{array}{rllrll}
tan\theta&=&\frac{y}{6}\qquad \qquad \qquad \qquad & tan\theta&=&\frac{y-x}{1.8}\\\\
y&=&6tan\theta}\qquad& y-x&=&1.8tan\theta\\\\
&&&x&=&y-1.8tan\theta\\\\
&&&x&=&6tan\theta -1.8tan\theta\\\\
&&&x&=&4.2tan\theta \\\\
\frac{dy}{d\theta}&=&6sec^2\theta}\qquad& \frac{dx}{d\theta}&=&4.2sec^2\theta\\\\
\end{array}$$

 ---------------------------------------------------------------------------------------------------------

              $$\begin{array}{rll}
\frac{dy}{dt}&=&\frac{dy}{d\theta}\times \frac{d\theta}{dx}\times \frac{dx}{dt}\\\\
\frac{dy}{dt}&=&6sec^2\theta}\times \dfrac{1}{4.2sec^2\theta}\times 1\\\\
\frac{dy}{dt}&=&\frac{6}{4.2}\\\\
\frac{dy}{dt}&=&\frac{10}{7}\\\\
\frac{dy}{dt}&=&1\frac{3}{7}\;\;m/s\\\\
\end{array}$$

 

b)  $$\frac{dy}{dt}-\frac{dx}{dt}= 1\frac{3}{7}-1=\frac{3}{7}\;\;m/s$$

 

So the end of the shadow is moving away at a constant rate of      $$1\frac{3}{7}m/s$$

and

The length of the shadow is changing at a constant rate of     $$\frac{3}{7}\;\;m/s$$

Since both of these are constants, it makes no difference how foar away from the pole the man is.

The answers will always be the same.     

Jul 20, 2014
 #1
avatar+3502 
+5
Jul 20, 2014
 #1
avatar+158 
+8
Jul 20, 2014

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