# Find shortest time to travel

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• May 22nd 2012, 05:44 PM
ineedhelplz
Find shortest time to travel
Firstly, apologies for the hard-to-see image. It's just for an outline of the problem.

The questions asks to find the value of x (on the right vertical side) to find the minimum time to travel to the top right corner of the "forest"

The bush can be travelled through at 3km/h and the "clear" area can be travelled through at 5km/h.

I started by equating the times of the 2 routes to take but came up with the wrong answer.

Any help is greatly appreciated.

The graphic shows 2 km at the bottom and x km on the right side.

Attachment 23934
• May 23rd 2012, 01:47 PM
Soroban
Re: Find shortest time to travel
Hello, ineedhelplz!

The diagram and description are not correct.
I think I've corrected them,

Quote:

Find the value of x (on the right vertical side) to find the minimum time to travel to the top right corner of the "forest".
The forest can be travelled through at 3 km/h and the "clear" area can be travelled through at 5 km/h.

The graphic shows 2 km at the bottom and x km on the right side.

Code:

                        * B -                         |  :                         |d-x:                         |  :                         *P  :                     *  |  d                 *      |  :               *        |x  :             *          |  :         *              |  :     A * - - - - - - - - *C  -       : - - -  2  - - - :
A man wants to get from A to B in the least amount of time.
. . $\displaystyle AC = 2,\;BC = d$

He walks through the "forest" from point A to point P
. . then walks in the "clear" from point P to point B.
Let $\displaystyle x = CP;\;d-x = PB.$

He walks distance $\displaystyle AP = \sqrt{x^2+4}$ at 3 km/hr.
. . This takes $\displaystyle \frac{\sqrt{x^2+4}}{3}$ hours.

He walks distance $\displaystyle PB = d-x$ at 5 km/hr.
., . This takes $\displaystyle \frac{d-x}{5}$ hours.

The total time is: .$\displaystyle T \:=\:\tfrac{1}{3}(x^2+4)^{\frac{1}{2}} + \tfrac{d}{5} - \tfrac{1}{5}x$

And that is the function we must minimize.,
• May 23rd 2012, 02:50 PM
ineedhelplz
Re: Find shortest time to travel
Thank you very much, that makes a lot of sense, however; how do I factor the extra distance d into the equation?

I get an answer of x=1.5 which is what I got initially. The answer in the solutions is 1.2km.

Could you please further complete this example? That would be a great help!

Thank you