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### Course: Congruence, Similarity, Right Triangles, & Trig 241+>Unit 3

Lesson 3: Congruent triangles

# Calculating angle measures to verify congruence

Calculate missing angle measures to determine which triangles must be congruent. Created by Sal Khan.

## Want to join the conversation?

• i don't have enough energy to watch these videos
😞🪫
• same here, but hey when i was here 2 monthes ago i only watch these videoes for energy points and i failed to finish even one video. then when i'm going to have a test on congruent triangles (which is next week), i realized the importance to learn through these useful videoes (even though i'm not a fan of sal) and that's how i've got the motivation to watch these at 8 pm.
hope this would help u a little
• Can someone please tell me an overview of this?? I am kinda confused. Thanks!
• Every triangle must add up to 180. If I have 2 angles that are 35 degrees each, I get 70 degrees total. When I subtract 70 from 180 I am left with 110 degrees which is the measure of the 3rd angle. Hope this helped!
• We know by ASA that they are congruent but can't we also use ASS to see that they are congruent? Or is there only one option?
• First off: I wouldn't suggest using that term, you may get flagged/get some rude comments. Use SSA instead.

Next: Check out a few of the previous videos, Sal goes over this to prove why SSA doesn't work. And for your last comment... ASA is not the only postulate. The triangle congruency postulates (as covered so far) are: SAS, ASA, AAS, and SSS.
• subtract the given angle from 180
• Why is there energy points in the video
(1 vote)
• Why does Sal always start his explanation with "one way to think about it"? Don't get me wrong, I think it's a very good way to start a explanation, as it doesn't set up his explanation as the only valid piece of reasoning, but I'm just curious.
(1 vote)
• That would be because his view is not the only valid one, just the one he chooses to teach here. I like that he acknowledges the reality that there are other methods used to solve these problems. Some users come from educational backgrounds that use different systems, like sometimes Sal teaches both methods and then asks which one your class uses.
• What about those numbers on the outside of the triangle? (Ex:6 on the <CBA> Triangle.)
(1 vote)
• Also can you tell the triangles are congruent by only their side angles?
(Ex: 2 Triangles have sides. The first triangle has 6,9,10. Second has 18,27,30. Are they congruent?)
(1 vote)
• yes and no because that is a dilation it would be similar but not congruent but if another triangle has 6,9,10 that would be congruent not 18,27,30
(1 vote)
• am I able to write the name of the triangle in any arrangmnent I want or is it specific
(1 vote)
• It is specific. For example Triangle ABC ≅ Triangle DEF, it means angle A = angle D, angle B = angle E, angle C = angle F. And side AB = DE etc.
(1 vote)
• how can you tell which two triangles are congruent if they all look the same
(1 vote)
• It is the angle numbers. With knowing all of the angles it is much easier figuring out which two triangles are congruent. And remember the size also has to be the same. Otherwise there is not much information to support the congruence of the triangles