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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.

## Want to join the conversation?

• What are complementary and supplementary angles?
• Good question!

Complementary angles are two angles that add up to 90°. For example, 30° + 60° = 90°, so 30° and 60° would be complementary angles.

Supplementary angles are similar: they're two angles that add up to 180°. For example, 60° + 120° = 180°, so 60° and 120° are supplementary angles.
• What are skew lines? I have heard of them before, but I forgot what they are.
• Skew lines are lines that neither intersect nor are parallel. The idea is to realize that they lie in different coordinate planes ;)
• 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?
• 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.
• 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!
• 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.
• So meaning to say Perpendicular Lines Intersect at a( 90 Degree) right angle, and Parallel lines never intersect.

Am i...right?
• You're correct. they don't intersect.
(1 vote)
• Why do perpendicular lines intersect? Why Can't parallel lines intersect?
• Parallel Lines share the same slope, but your wording is a bit incorrect. Parallel Lines can intersect, if and only if the two lines have infinite intersection (So one line overlay on another).

All other lines that don't share the same slope intersects at some point (Assuming 2D) because those two lines form an angle at the intersection.
• What are lines that intersect, but aren't perpendicular called
• They are called the skew lines, they contain most of the lines found in the coordinate graph. Their properties are basically intersect but not at a right angle
• What are complementary and supplementary angles?
• Complementary angles:
- two angles that add up to 90°
- example: 10° & 80° are complementary

Supplementary angles:
- two angles that add up to 180°
- example: 5° & 175° are supplementary

Hope this helps!