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## High school geometry

### Course: High school geometry>Unit 3

Lesson 7: Proofs of general theorems

# Geometry proof problem: midpoint

Sal proves that a point is the midpoint of a segment using triangle congruence. Created by Sal Khan.

## Want to join the conversation?

• I am really confused by this entire proof. Can anyone explain it?
• what did he say that AB hade the same segment as CD would that make the statement false?
• No, not necessarily. They are linked with 2 line segments, and they are BOTH line segments, so no, that doesn't automatically declare it false.
Hope this helped.
• Why do you put a tilde "~" over the equal sign when doing proofs? Why do you need to qualify it with "approximately equal to"?
• Firstly ~doesn't mean approximately equal to and secondly if we just use = sign than it just clarify the properties to be same but if we use~sign than it would mean that an object is same to another object in respect of shape size and all,
• Do vertical angles have to be ALWAYS congruent?
• How do you define transversal? (Spelled it wrong?)
• Transversal: A line that crosses at least two other lines.
(1 vote)
• When I took Geometry in HS, a looooong time ago, our math instructor provided us with a corollary called CPCFE, Corresponding Parts of Congruent Figures are Equal. Using it, you could conclude that corresponding legs of congruent triangles were equal. Is that old school? I don't see Sal using it anywhere in his lessons.
• I took geometry last year, and used CPCTC ALL the time. It is so useful. Actually, Mr. Khan did use it, he just didn't put it as a step. At , He said that if we know that the triangles are congruent, the corresponding angles are congruent. That is because of CPCTC, or CPCFE, as you called it.
• If at , I wanted to prove the remaining angles congruent (m angle BAE = m angle CDE) would I also name them alternate interior angles?

Also, just to be certain, would this be enough to prove the triangles congruent? Thank you.
• Why did he keep talking about a 'green' color when there was no green?
• I know what you mean, he is talking about a yellow-green. It appears differently on his screen
• How do you know in which order to do a proof?