Homework Help: How to use the graph of the distance between any two points

1. Feb 9, 2010

Juwane

1. The problem statement, all variables and given/known data

Write an expression for the distance between the point P(1,2) and an arbitrary point $$( x, \sqrt{x} )$$ on the curve $$y = \sqrt{x}$$. Graph this distance versus x, and use the graph to find the x-coordinate of the point on the curve that is closest to the point P.

2. Relevant equations

N/A

3. The attempt at a solution

Well, here's the expression I wrote for the distance:

$$d = \sqrt{ (x - 1)^2 + ( 2 - \sqrt{x} )^2 }$$

I've graphed this on a graphic software. Now, how can I use this graph to answer the question? What do I have to look for on the graph?

2. Feb 9, 2010

Mentallic

Since this is the in the pre-calculus forum I'll assume you won't be using derivatives, so you'll be finding an approximate solution to the shortest distance.

When you graphed the distance versus x, at what (approx) x value is the distance the shortest? In other words, where is d the smallest?

3. Feb 9, 2010

Juwane

d is smallest when x is approx equal to 1.35296

The answer is correct as given at the back of the book.

Yes, I didn't want to use derivatives for this question; but if I were to use derivatives, I would have differentiated the function and equated it to zero, and then would've solved for x, right?

EDIT: Also note that in the graph there's only one minimum and no maximum extrema, so differentiating twice won't be necessary.

Last edited: Feb 9, 2010
4. Feb 9, 2010

Mentallic

Yes that's correct