# Intersection of 2 Perpendicular Lines - Apparent Error in Intersection Coordinates

1. ## Intersection of 2 Perpendicular Lines - Apparent Error in Intersection Coordinates

I have plotted an arbitrary line y=0.83333*x +3. I needed a 2nd Line perpendicular to this going through the Origin (0,0). To be perpendicular that line needs a slope of -1*1/0.83333 = -1.2. That will be y = -1.2*x. I have used the formula as in the Diagram attached and get a a point of intersection. But this point is apparently wrong as it could be seen in the diagram. Any explanation?
Int.JPG

2. ## Re: Intersection of 2 Perpendicular Lines - Apparent Error in Intersection Coordinates

Adding to my question above, this is what I get from Geogebra...
Geo.JPG

3. ## Re: Intersection of 2 Perpendicular Lines - Apparent Error in Intersection Coordinates

How did you get your x value? The intercept formula is roughly -3/2 which is not -0.67. I think you inverted the division.

4. ## Re: Intersection of 2 Perpendicular Lines - Apparent Error in Intersection Coordinates

Thank you very much rorya. When I invert the formula it works. I had taken the formula from this link: https://www.listendata.com/2012/10/e...wo-linear.html. The same formula could be found in many other places and fora too. That is why I am puzzled. Are they all wrong?

Here is the result when I invert the formula:
Early_Problem.JPG

5. ## Re: Intersection of 2 Perpendicular Lines - Apparent Error in Intersection Coordinates

No, the formula you posted was correct: (int2 - int1)/(slope1 - slope2)

which is: (0-3)/(0.83333-1.2) = -3/2.033333 = -1.47541

What you appear to have calculated was: (slope1 - slope2)/(int2 - int1)

which is: 2.033333/-3 = -0.67778

6. ## Re: Intersection of 2 Perpendicular Lines - Apparent Error in Intersection Coordinates

That is correct. Brilliant. I have inadvertently inverted the formula.

I will mark this answer as correct and the question [Solved] and add 1 to your reputation.

Thank you very much.

7. ## Re: Intersection of 2 Perpendicular Lines - Apparent Error in Intersection Coordinates

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