# word problem and derivatives

• Sep 9th 2009, 08:55 PM
yoman360
word problem and derivatives
a ladder 10 ft long rests against a vertical wall. if the bottom of the ladder slides away from the wall at a speed of 2 ft/s how fast is the angle between the top of the ladder and the wall changing when the angle is pi/4 radians?
• Sep 9th 2009, 09:40 PM
yoman360
this one is difficult
• Sep 9th 2009, 09:59 PM
seld
I might be wrong but I think this is right.

If you think of the angle $\theta$ the only thing way of measuring it is by the hypotenuse. So cosine is useful, but how do you find the adjacent side?

well the hypotenuse = 10

the adjacent side is some value let's say C plus it's growing by 2ft/sec. So mathematically that means:

$2\frac{\text{ft}}{\text{sec}}*\text{time}+ C$

aka: $2t+C$

so then: $cos\theta=\frac{2t+C}{10}$

So now you want to see how much the angle theta is changing as time goes by, so you take the derivative in terms of t.

$\frac{d}{dt}(cos\theta)=\frac{d}{dt}(\frac{2t+C}{1 0})$

Can you take it from there?