If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

### Course: Geometry (all content)>Unit 5

Lesson 2: Quadrilateral proofs & angles

# Whether a special quadrilateral can exist

Proving whether a special quadrilateral can exist or not. Created by Sal Khan.

## Want to join the conversation?

• Can a quadrilateral have a one-hundred eighty degree angle in it and have the 180 degree angle be the perpendicular bisector? I think that it would make the quadrilateral that he discusses in this video possible.
• If a 'quadrilateral' had one of its four angles = 180 degrees, then it would be a triangle.
• it violates the condition of triangle, external angleAEB is sum of internal opposite angles. so it cannot be equal to angleECB....unless angle EBC becomes zero...this makes your case, lines l and m become parrell..m i right
• does square have the property of being a special quadrilateral?
• yes it does

a square is a rectangle because a rectangles has two sets of parallel sides a nd so does a square! but a rectangle can't be a square because its sides are not all equal, so in a way a square is special
• If you turn that quadrilateral is it a kite?
• At an assumption is made to prove if it is true or false the statement what is it called when you do this?
• It is a classic proof strategy. Lets say your given this question:
Ken's dog does not have a tail.
So for the sake of proof's, lets say that this is correct (just go with it).
If Ken's pet is a dog, the it has a tail since dog's have tails. So, this statement is wrong.
This is called a indirect proof.
Hope this helps :D
• A quadrilateral is just a polygon with four sides. That is why it is called a "quadrilateral", since "quad" means "four".