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Parallel & perpendicular lines intro

Parallel lines never intersect, and perpendicular lines intersect at a 90 degree angle. Learn how to identify parallel and perpendicular lines. Created by Sal Khan.

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  • aqualine ultimate style avatar for user TheEpicOne
    What are complementary and supplementary angles?
    (154 votes)
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  • orange juice squid orange style avatar for user William Carroll
    What are skew lines? I have heard of them before, but I forgot what they are.
    (50 votes)
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  • hopper cool style avatar for user maceden
    What about two lines that don't intersect where you see them, but are placed at an angle so if they continued to move in their same direction they would intersect? What kind of line/angle is that?
    (29 votes)
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    • female robot grace style avatar for user Anna
      it depends on where exactly they intersect and their slope. If 1 slope is 1/2 and another is -2 than there would be all right angles so it is perpendicular.

      If it doesn't follow that 1 line has a slope that is the negative of the reciprocal of the other line's slope but they don't have the same slope than they are just considered to be intersecting lines.
      (26 votes)
  • aqualine ultimate style avatar for user Mitchtacular
    So meaning to say Perpendicular Lines Intersect at a( 90 Degree) right angle, and Parallel lines never intersect.

    Am i...right?
    (8 votes)
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  • purple pi teal style avatar for user Noahjrkingsb
    Why do perpendicular lines intersect? Why Can't parallel lines intersect?
    (5 votes)
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  • marcimus pink style avatar for user Vasundhara Biswas
    When Sal Khan sir says at that those lines he just drew can't be called perpendicular or parallel but they just intersect, then why can't we call them intersecting lines? Silly question you might think maybe , but I'm just curious!
    (4 votes)
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    • duskpin sapling style avatar for user proxima
      You can call them intersecting lines. Perpendicular lines are just a more specific type of intersecting lines that have special properties and because of wanting to identify these properties (for proofs or anything else) and be specific, we call those lines perpendicular lines. In some cases, like the one Sal drew at , intersecting lines is the most specific you can get and there is nothing wrong with calling them that.
      (6 votes)
  • hopper cool style avatar for user Max
    What are complementary and supplementary angles?
    (3 votes)
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  • aqualine ultimate style avatar for user phoenix122
    What are lines that intersect, but aren't perpendicular called
    (5 votes)
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  • marcimus pink style avatar for user Natalia F.
    Ok, so parallel is two lines that never cross, and perpendicular is two lines that make a right angle? and what are Complementary and Supplementary angles?
    (4 votes)
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  • blobby green style avatar for user cordobahadith
    cool man you know some long words ok here is my question what if 2 lines are not straight but are perpendicular? what u call them?
    (3 votes)
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Video transcript

Let's think a little bit about two terms that you'll see throughout your geometry, and really, mathematical career. One is the idea of things being perpendicular. And usually, people are talking about perpendicular. Actually I'm misspelling it-- perpendicular lines, and the idea of parallel lines. So perpendicular lines are two lines that intersect at a right angle. So what am I talking about? So let's say that this is one line right over here and that this is another line right over here. We would say these two lines are perpendicular if they intersect at a right angle. So they clearly intersect. In order for them to intersect at a right angle, the angle formed between these two lines needs to be 90 degrees. And if any one of these angles is 90 degrees, the rest of them are going to be 90 degrees. So this is 90 degrees, then these are perpendicular lines. And if that's 90 degrees, then that's going to be 90 degrees, that's going to be 90 degrees, and that's going to be 90 degrees. So if any of them are 90 degrees, the rest of them are 90 degrees, and we have perpendicular lines. If you have two lines that on a two-dimensional surface like your paper or like the screen never intersect, they stay the same distance apart, then we are talking about parallel lines. So this line right over here and this line right over here, the way I've drawn them, are parallel lines. They aren't intersecting. They're both kind of going in the same direction, but they're kind of shifted versions of each other. They will never intersect with each other. So these two are parallel. If we have two lines that, let's say, they intersect, but they don't intersect at a right angle, so let's say we have that line and we have this line right over here, and they're clearly not intersecting at a right angle, then we call these neither perpendicular nor parallel lines. These lines just intersect.