Parallel Line Formula: Solving for y=kx+n with Point (-4,-5)

[Physicsissuef]

Homework Statement

Can somebody use the formula y=kx+n, to present me formula for the line which is parallel to the y-axis and go among the point (-4,-5)?

Homework Equations

y=kx+n

The Attempt at a Solution

k=tg(alpha)

k=tg90

k=$$\frac{1}{0}$$

y=$$\frac{x}{0}$$

x=0

-4=0

Hm...

 

[Feldoh]

If the line is parallel to the Y AXIS then the slope has to be undefined, see why?

 

[rock.freak667]

Homework Statement

Can somebody use the formula y=kx+n, to present me formula for the line which is parallel to the y-axis and go among the point (-4,-5)?

Homework Equations

y=kx+n

The Attempt at a Solution

k=tg(alpha)

k=tg90

k=$$\frac{1}{0}$$

y=$$\frac{x}{0}$$

x=0

-4=0

Hm...

you can't multiply by zero like that.

If the line is parallel to the Y AXIS then the slope has to be 0, see why?

The y-axis is the vertical one,right?

 

[sutupidmath]

wouldn't that be just x=-4?

 

[rock.freak667]

That's what I believe it will be.

 

[Physicsissuef]

I know that it will be x=-4. But please prove it with y=kx+n
rock.freak667, y is the vertical one.

 

[kamerling]

The problem is that the two assumptions

k, and n exist such that {(x,y): y=kx+n} is the set of points of the line

{(x,y): x = -4} is the set of points of the line

can not both be true.

 

[Physicsissuef]

what is k, what is n?

 

[HallsofIvy]

I know that it will be x=-4. But please prove it with y=kx+n
rock.freak667, y is the vertical one.

You can't. A vertical line, parallel to the y-axis, cannot be written in that form.

 

[Physicsissuef]

You can't. A vertical line, parallel to the y-axis, cannot be written in that form.

Why?

 

[HallsofIvy]

For exactly the reason everyone has been telling you! Writing "y= kx+ n" means that different values of x give different values of y. That is true for all lines except vertical lines. A line parallel to the y-axis has the same value of y for every x. When you learned "y= kx+n", the "slope-intercept" form, you should have learned that every non-vertical line can be written in that form but vertical lines cannot.

 

[Physicsissuef]

Ahhh... I undertand now. Thank you very much.
btw- what is that same value for y?

 

[HallsofIvy]

My mistake. I meant to say the "same x value for every y". In this case, since you tell us one point is (-4, -5) that x value is x= -4 for all y. That's why we can write the equation of that line "x= -4" without mentioning y.

(A horizontal line would have the "same y value for every x". The horizontal line (parallel to the x-axis) passing through (-4, -5) would have y= -5 for every y. Of course, that is of the form "y= kx+ n" with k= 0, n= -5.)