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Perpendicular lines from equation

Sal determines which pairs out of a few given linear equations are perpendicular. Created by Sal Khan and Monterey Institute for Technology and Education.

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  • blobby green style avatar for user Heather Howton
    What is an equivalant line? It says in the example I have a choise between perpendicular lines, paralel lines, equivalant lines, and so forth. Thank You!
    (45 votes)
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    • leaf green style avatar for user Gemma Hyeon
      Equivalent lines are what they say, the same lines! Let's say you're given this example: 5x+y=2 and 20x+4y=8 and you have to figure out what kind of line this is. This would be an equivalent line because you can make the 2 equations the same by multiplying 5x+y=2 by 4 to get the other equation 20x+4y=8 so they are equivalent lines. I really stink at explaining stuff, unlike Sal but I tried my best and I hope this helped! :)
      (90 votes)
  • male robot hal style avatar for user Ibrahim
    Could you prove that slopes of perpendicular lines are negative inverses of each other using trigonometry?
    (15 votes)
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    • aqualine sapling style avatar for user Jesús Redondo Pascual
      Yes, you can use triangles to prove the slopes are inverses.
      Imagine a line from 0,0 to 3,4. At 3,4 you can draw a right triangle with the axis and the point 0,0.
      Now imagine a perpendicular line, that intersect at the point 3,4. You can make another triangle from 3,4 to the axis.
      The 2 triangles are complementary because the 2 acute angles at the intersection sum 90 degrees, and the 2 triangles have a right angle.
      So the relationship between the sides of the 2 triangles is the inverse. And, if the relationship between the sides is the inverse, the slopes of the triangles are inverse also.
      But also negative!!! Why? I don't use trigonometry but it's pretty simple.
      If a line goes up, positive slope, the perpendicular goes down, negative slope. If a line goes down, negative slope, the perpendicular goes up, positive slope.
      (16 votes)
  • male robot hal style avatar for user Chris Hohmann
    Is there a more rigid definition for perpendicularity? A line of the form y = C has a slope of 0. There is no negative reciprocal for 0, but a line of the form x = C is still perpendicular.
    (8 votes)
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    • male robot hal style avatar for user Chris Hohmann
      Thanks for the feedback. The problem with your definition is that it does not cover the case for a horizontal and vertical line which are perpendicular, but the product of their slopes are 0, not -1. I think the wikipedia definition I found covers this limit case.
      (2 votes)
  • leafers ultimate style avatar for user Patrick Lewis
    At Sal tells me that perpendicular lines must intersect with a right angle.
    Parallel lines never meet because they have the same slope...
    So what exactly is it when to lines cross, but not with 90 degree angle?
    (6 votes)
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  • blobby green style avatar for user Michelle Wellman
    In a square can there be perpendicular line segments?
    (8 votes)
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  • leaf green style avatar for user Keerthi
    Do perpendicular lines have to have the same Y intercept? Please help ASAP!
    (6 votes)
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  • hopper cool style avatar for user Quantum Coding
    If you have two equations of lines, how do you tell if they even intersect, not just if they're perpendicular?
    (3 votes)
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    • piceratops tree style avatar for user Mo Acty
      You look at the slope (which either is clearly visible in the equation or you can compute it easily from the coefficients of x and y in the equation). If slopes are the same then the lines are either equivalent (both equations describe the same line) or parallel (and thus do not intersect).

      Two lines with different slopes will always intersect.
      (3 votes)
  • starky sapling style avatar for user peroniakd
    How do I know such the equation is or not a Perpendicular line from the equation? What number the slope need be? It isn't the inverse, right?
    (2 votes)
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  • mr pants teal style avatar for user pupatel
    If it doesn't intersect at a 90 degree angle are they still perpendicular? Thanks
    (1 vote)
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  • blobby green style avatar for user ISSAM ELMOULOUDI
    can you give me an example about the perpendicular equation from the graph
    (2 votes)
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Video transcript

We are asked which of these lines are perpendicular. And it has to be perpendicular to one of the other lines, you can't be just perpendicular by yourself. And perpendicular line, just so you have a visualization for what for perpendicular lines look like, two lines are perpendicular if they intersect at right angles. So if this is one line right there, a perpendicular line will look like this. A perpendicular line will intersect it, but it won't just be any intersection, it will intersect at right angles. So these two lines are perpendicular. Now, if two lines are perpendicular, if the slope of this orange line is m-- so let's say its equation is y is equal to mx plus, let's say it's b 1, so it's some y-intercept-- then the equation of this yellow line, its slope is going to be the negative inverse of this guy. This guy right here is going to be y is equal to negative 1 over mx plus some other y-intercept. Or another way to think about it is if two lines are perpendicular, the product of their slopes is going to be negative 1. And so you could write that there. m times negative 1 over m, that's going to be-- these two guys are going to cancel out-- that's going to be equal to negative 1. So let's figure out the slopes of each of these lines and figure out if any of them are the negative inverse of any of the other ones. So line A, the slope is pretty easy to figure out, it's already in slope-intercept form, its slope is 3. So line A has a slope of 3. Line B, it's in standard form, not too hard to put it in slope-intercept form, so let's try to do it. So let's do line B over here. Line B, we have x plus 3y is equal to negative 21. Let's subtract x from both sides so that it ends up on the right-hand side. So we end up with 3y is equal to negative x minus 21. And now let's divide both sides of this equation by 3 and we get y is equal to negative 1/3 x minus 7. So this character's slope is negative 1/3. So here m is equal to negative 1/3. So we already see they are the negative inverse of each other. You take the inverse of 3, it's 1/3, and then it's the negative of that. Or you take the inverse of negative 1/3, it's negative 3, and then this is the negative of that. So these two lines are definitely perpendicular. Let's see the third line over here. So line C is 3x plus y is equal to 10. If we subtract 3x from both sides, we get y is equal to negative 3x plus 10. So our slope in this case is negative 3. Now this guy's the negative of that guy, this guy's slope is a negative, but not the negative inverse, so it's not perpendicular. And this guy is the inverse of that guy but not the negative inverse, so this guy is not perpendicular to either of the other two, but line A and line B are perpendicular to each other.