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## Integrated math 1

### Course: Integrated math 1>Unit 18

Lesson 4: Parallel & perpendicular lines on the coordinate plane

# 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.
• Help me get 20 upvotes :) pls
• So meaning to say Perpendicular Lines Intersect at a( 90 Degree) right angle, and Parallel lines never intersect.

Am i...right?
• Yes, you are right about that. Perpendicular lines intersect at 90 degrees, but parallel lines never intersect because they never touch.
• Why do perpendicular lines intersect? Why Can't parallel lines intersect?
• Parallel lines never touch or point at each other. However, perpendicular lines intersect because they touch each other.
• 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.
• 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!