Do dogs know calculus? Problem 64 of Calculus Early Transcendentals Second Edition.

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A mathematician stands on a beach with his dog at point A. He throws a tennis ball so that it hits the water at point B. The dog, wanting to get to the tennis ball as quickly as possible, runs along the straight beach line to point D and then swims from point D to point B to retrieve his ball. Assume C is the point on the edge of the beach closest to the tennis ball.

a. Assume the dog runs at speed r and swims at speed s, where r is greater than s and both are measured in meters/seconds. Also assume the lengths of BC,CD,and AC are x,y,and z, respectively. Find a function T(y) representing the total time it takes the dog to get to the ball.

b. Verify that the value of y that minimizes the time it takes to retrieve the ball is y=x/squareroot(r/s +1) x squareroot(r/s-1)


you can find the original article that inspired this problem at http://www.maa.org/sites/default/files/pdf/features/elvisdog.pdf.

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Explanation & Answer

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Calculus
Part a
Consider the time function of the dog to get the ball is T (y)
Now, consider the running time. By running it passes AD. So, you can say:
AD=AC-DC
Put DC=y and BC=x
AD=z-y
The running speed is r m/s
So, the time function for this is;
𝑇1 (𝑦) ...


Anonymous
Just the thing I needed, saved me a lot of time.

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