MHB Dexter's question at Yahoo Answers regarding related rates

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The discussion focuses on solving a related rates problem involving an observer, an elevator, and the angle of elevation. The relationship between the angle, height of the elevator, and horizontal distance is established using trigonometric functions. By differentiating the equation with respect to time, the rate of change of the angle is derived. Specific values are plugged into the final formula to calculate the rate of change of the angle at two different heights. The results indicate how the angle of elevation changes as the height of the elevator varies.
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Here is the question:

View attachment 886

I have posted a link to this topic so the OP can see my work.
 

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Hello Dexter,

I would let $E$ be the elevation of the observer, $h$ be the height of the elevator above the ground, and $w$ be the horizontal distance of the observer from the elevator shaft.

From the diagram, we see we mat then state:

$$\tan(\theta)=\frac{h-E}{w}$$

Differentiating with respect to time $t$ (recognizing that $\theta$ and $h$ are the only variables that change with time), we find:

$$\sec^2(\theta)\frac{d\theta}{dt}=\frac{1}{w}\frac{dh}{dt}$$

Multiplying through by $$\cos^2(\theta)$$, we get:

$$\frac{d\theta}{dt}=\frac{\cos^2(\theta)}{w}\frac{dh}{dt}$$

By Pythagoras, we know:

$$\cos^2(\theta)=\frac{w^2}{(h-E)^2+w^2}$$

and thus we may state:

$$\frac{d\theta}{dt}=\frac{w}{(h-E)^2+w^2}\frac{dh}{dt}$$

Plugging in the given data, we have (in radians per second):

$$\frac{d\theta}{dt}=\frac{54}{(h-21)^2+18^2}$$

And so to answer the questions, we find:

$$\left.\frac{d\theta}{dt} \right|_{h=15}=\frac{54}{(15-21)^2+18^2}=\frac{3}{20}$$

$$\left.\frac{d\theta}{dt} \right|_{h=39}=\frac{54}{(39-21)^2+18^2}=\frac{1}{12}$$
 
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