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### Course: Class 9>Unit 13

Lesson 3: Triangles 7.3

# Geometry proof problem: congruent segments

Sal proves that two pairs of segments are congruent using the ASA and AAS congruence criteria. Created by Sal Khan.

## Want to join the conversation?

• What is a chord?
• A chord is a line segment within a circle that touches 2 points on the circle.
• if you don't put a congruence symbol, then you're supposed to put an "m" in front of an angle statement ( m<A=m<C), right?
• yes, it stands for "the meausre of angle A = to the measure of angle C
• But how would you calculate the angle of a congruent triangle's angle when it isn't given on the other side, like adding/multiplying/deviding/subtracting from surrounding angles to find the angle of x, like in some of the Practices on the learning dashboard? I can't find a video for that |:(|)
• In the Congruent Triangles 2 problem set, you are still using the ideas covered in this set of videos (plus the "triangle angles sum to 180" and angle congruency rules). So when you are trying to figure out what x is try these common approaches:
1) see if it is equal to any of the angles you already have, maybe through vertical angles, for instance.
2) see if you can calculate it through the triangle-sum=180 rule - if you have the other two angles in the triangle, subtract them from 180 to get your angle
3) see if the other triangle in the diagram is congruent. If you have matching sides and angles enough to say the two triangles are congruent, then you can match them (carefully, so the correct angles/sides align) and find out what x is by grabbing it's congruent value.

As they get trickier, you will have to combine these approaches to get an answer.

Hope that helps!
• Can't you just do a rotation then a translation? And you get the same solution? And make it a whole lot easier??
• can you say ctcpc as the reason for #5
(1 vote)
• Yes, because you proved that the triangles are congruent in the previous step, so therefore, everything thing is congruent in both triangles
• He took almost 12 minutes trying to explain why the triangles are congruent, why so long?
• Longer videos are generally more comprehensive and provide in-depth answers that demonstrate how to solve these types of problems.
• At , Sal says that we now know that BF and CE are equal because Corresponding sides are congruent but I don't get how he got that from the last statement.
• because he was able to show that triangle BAF= triangle CAE he could then determine that BF=CE because they are both the bases of two triangles that are the same size.
(1 vote)
• I appreciate all of you so much. Thank you for this gift of teaching.
I do have a concern and a suggestion. In this vid at you've placed one of the typical corrections. I would suggest instead rerecording that segment with an edited correction.
So often I wish we could shorten the videos by eliminating rewriting to change colors or to correct previous statements. I want to learn so much from you that I need instruction to be as correct and efficient as possible.