# Homework Help: Finding a point on a line, given another point on the same line, and knowing its dist

1. Jun 8, 2010

### Telemachus

Hi there. I'm tryin to find a point, lets call it C. I'm working on a R². What I know is that the point belongs to the line L: $$y=\displaystyle\frac{x}{2}+\displaystyle\frac{1}{2}$$ And that the distance to the point B(1,1), that belongs to L is $$\sqrt[ ]{20}$$.

How can I find it? I know there are two points, cause of the distance over the line.

I've tried to solve it using the distance pythagoric equation, but I don't know how to use the fact that B and C belongs to the same line.

2. Jun 8, 2010

### Staff: Mentor

Re: Finding a point on a line, given another point on the same line, and knowing its

Every point on line L has coordinates (x, x/2 + 1/2). Set up an expression that represents the distance between this point and (1, 1), and set that expression to sqrt(20). Then solve the equation for x.

You should get two values for x, since there are two points on the line that are sqrt(20) units away from (1, 1).

3. Jun 8, 2010

### Telemachus

Re: Finding a point on a line, given another point on the same line, and knowing its

Thanks Mark44. Heres my attempt to solve it:

$$\sqrt[ ]{20}=\sqrt[ ]{(1-x_0)^2+(1-y_0)^2}$$

$$(1-x_0)^2+(1-y_0)^2=20$$

So, I know that for any value of $$x_0$$, $$y_0$$ must be $$y_0=x_0/2 + 1/2$$ and I know $$(1-x_0)^2+(1-y_0)^2=20$$

Solving the system should I get the two values?

Bye there, and thanks again.

4. Jun 8, 2010

### Staff: Mentor

Re: Finding a point on a line, given another point on the same line, and knowing its

Well, you can do that in one equation.
$$\sqrt{20}=\sqrt[ ]{(1-x_0)^2+(1-x_0/2 - 1/2)^2}$$

Yes, you should get two values for x0.

5. Jun 9, 2010

### Telemachus

Re: Finding a point on a line, given another point on the same line, and knowing its

Thank you.