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Formula y=kx+n help

  • #1
1. 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)?

2. Homework Equations

y=kx+n

3. The Attempt at a Solution

k=tg(alpha)

k=tg90

k=[tex]\frac{1}{0}[/tex]

y=[tex]\frac{x}{0}[/tex]

x=0

-4=0

Hm.....
 

Answers and Replies

  • #2
1,341
3
If the line is parallel to the Y AXIS then the slope has to be undefined, see why?
 
Last edited:
  • #3
rock.freak667
Homework Helper
6,230
31
1. 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)?

2. Homework Equations

y=kx+n

3. The Attempt at a Solution

k=tg(alpha)

k=tg90

k=[tex]\frac{1}{0}[/tex]

y=[tex]\frac{x}{0}[/tex]

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?
 
  • #4
1,631
4
wouldn't that be just x=-4?
 
  • #5
rock.freak667
Homework Helper
6,230
31
That's what I believe it will be.
 
  • #6
I know that it will be x=-4. But please prove it with y=kx+n
rock.freak667, y is the vertical one.
 
  • #7
454
0
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.
 
  • #8
what is k, what is n?
 
  • #9
HallsofIvy
Science Advisor
Homework Helper
41,738
899
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.
 
  • #10
You can't. A vertical line, parallel to the y-axis, cannot be written in that form.
Why?
 
  • #11
HallsofIvy
Science Advisor
Homework Helper
41,738
899
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.
 
Last edited by a moderator:
  • #12
Ahhh... I undertand now. Thank you very much.
btw- what is that same value for y?
 
  • #13
HallsofIvy
Science Advisor
Homework Helper
41,738
899
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.)
 

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