minimization problem, a y-shaped wire

[Joseph007]

Centerville is the headquarters of Greedy Cablevision Inc. The cable company is about to expand service to two nearby towns, Springfield and Shelbyville. There needs to be cable connecting Centerville to both towns. The idea is to save on the cost of cable by arranging the cable in a Y-shaped configuation. Centerville is located at (9,0) in the xy-plane, Springfield is at (0,4), and Shelbyville is at (0,- 4). The cable runs from Centerville to some point (x,0) on the x-axis where it splits into two branches going to Springfield and Shelbyville. Find the location (x,0) that will minimize the amount of cable between the 3 towns and compute the amount of cable needed. Justify your answer.

To solve this problem we need to minimize the following function of x:
f(x)=_________

We find that f(x) has a critical number at x=_________

To verify that f(x) has a minimum at this critical number we compute the second derivative f''(x) and find that its value at the critical number is ________ a positive number.

Thus the minimum length of cable needed is _________

I have no clue where to start, as in finding the appropriate equation. After that, I think I can get it.

 

[skeeter]

let the split be at (x,0)

distance from (9,0) to (x,0) = 9-x

let d = distance from (0,4) to (x,0) = distance from (0,-4) to (x,0) = ???
... use the distance formula between points in a plane to determine d as a function of x.

total length of cable, C(x) = (9 - x) + 2d, where d is what you determined from above.