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This is a multiple choice question in one of tests I just wrote and I did not know the answer to it. I was just stuck on this during the test. It is a very weird question, one I find to be impossible.

Here it goes:

Scientists have determined some species of birds tend to avoid flights over large bodies of water during daylight hours. It is believed that more energy is required to fly over water than land because air rises over land and falls over water during day. A bird with these tendencies is released from an island that is 5 km form the nearest point B on a straight shoreline, flies to point C on the shoreline, and then flies along the shoreline to its nesting area D. Assume that the brid instinctively chooses a path that will minimize its energy expenditure. Points B and D are 13 km apart.

If it takes 2 times as much energy to fly over water as land, what is the minimum amount of total energy that the bird could expend returning to its nesting area?

a) 2.88 units $\phantom{abcd}$ b) 21.66 units $\phantom{abcd}$ c) 23.00 units $\phantom{abcd}$ d) 27.85 units

I have no idea how to do this question because it makes absolutely no sense to me. I have sat and thought but I just am not getting anywhere.

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    That is exactly what I am thinking....energy is joules what the heck are they referring to :S2011-11-30

2 Answers 2

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diagram

This is a minimization problem, and you have to minimize

$E(x) = 2\cdot |AC| + |CD|$

where $x = |BC|$, with $C \in[B,D]$, $A\hat{B}D=90°$, $|AB|=5$ and $|BD|=13$.

That is,

$E(x) = 2 \cdot \sqrt{5^2+x^2}+13-x$

Take the derivative of this and find the zeros in $[0, 13]$, etc., giving the multiple choice answer (b).

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    @ratchet freak: Because of [this discussion](http://meta.math.stackexchange.com/questions/3271/are-uploaded-images-preserved-in-the-se-system), I edited your answer to put the definition of $x$ in text, in case the image-link expires (as apparently is likely, eventually). I hope you don't mind my doing these edits myself.2011-11-30
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Let $x$ be the distance from where the bird starts to $C$. Using the Pythagorean Theorem, one finds that the distance from $C$ to $D$ is $13-\sqrt{x^2-25}$ . Setting the energy expenditure factor over the shore equal to $a\,{\rm units\over km}$, the energy used is $ E(x)=2ax + a \cdot(13-\sqrt{x^2-25}). $

What you need to do is find the minimum value of $E$ over the interval $[5,\sqrt{194}]$ (if the bird first flies to $B$, then $x=5$ ; if the bird flies straight to $D$, then $x=\sqrt{194}$).

So:

Set E'(x)=0 and find any solutions in $[5,\sqrt{194}]$.

Evaluate $E$ at the solutions found above, at $x=5$, and at $x=\sqrt{194}$.

Select the smallest of these numbers. This will give you the minimum energy expenditure if the energy expenditure over land is $a\,{\rm units\over km}$.

By the way, the problem seems ill-posed, since they did not tell you what the energy expenditure over land was (I presume they took $a=1$).

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    I made exactly the same mistake the first time through! 13 is the distance from $B$ to $D$, not the distance from the island to $D$. So your twelves should be thirteens, and your thirteeens should be $\sqrt {194}$s.2011-11-30