009A Sample Final 2, Problem 9 Detailed Solution

A plane begins its takeoff at 2:00pm on a 2500-mile flight. After 5.5 hours, the plane arrives at its destination. Give a precise mathematical reason to explain why there are at least two times during the flight when the speed of the plane is 400 miles per hour.

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Background Information:
Intermediate Value Theorem
Let  $f(x)$  be a continuous function on the interval  $[a,b]$  and
without loss of generality, let  $f(a)<f(b).$
Then, for every value  $y,$  where $f(a)<y<f(b),$
there is a value  $c$   in  $[a,b]$  such that  $f(c)=y.$

Solution:

Step 1:
On average the plane flew
${\frac {2500{\text{ miles}}}{5.5{\text{ hrs}}}}\approx 454.5{\text{ miles/hr}}.$

Step 2:
In order to average this speed, the plane had to go from 0mph, up to full speed, past 454.5mph, and then it had to go back down to 0mph to land.
This means that there will be at least two times where the speed of the plane is 400mph by the Intermediate Value Theorem.

Final Answer:
See above.

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