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### Course: High school geometry>Unit 3

Lesson 5: Working with triangles

# Finding angles in isosceles triangles (example 2)

Sal combines what we know about isosceles triangles and parallel lines with the power of algebra to solve the angles of an isosceles triangle. Created by Sal Khan.

## Want to join the conversation?

• What does angle adjacent mean?
• yes s like that it's means the adjacent is next to one another
• Who's opening the door?!
• I really want to do my work, I really do but I cant bring myself to do it, I just feel like watching everything fall apart and not do anything about it, but I know that wont help me. I just wanted to get that out.
• It is ok Rose Sajaev. It happens to me too. Whenever I have that mindset, I breath in and ask myself, what would my mother do? How would she solve this problem? What is her mindset? I think that might help you. I would also recommend talking to a close friend about that. Do you know what else I do? Sometimes I have a flash back then a flash forward. I have a childhood memory in my head triggered by something I hate. Then, there is an image that I will see in the future. I do and I don't hate it. My flash forward images never tell me whenever I might get in trouble. You are not alone; I have a lot of friends who would say the same thing. Talk to me if you want.
• in what job will I have to find the measurement of an x variable. granted I want to be an engineer but when will I be given an equation like this?
• Engineers have to find the equations themselves and be able to solve them. And they will be much, much more advanced than this material.
• I don't get the proofs, can someone explain all of them to me, please? please answer me!
• Okay. Proofs are a way to show that the statement that you put on the paper is true. There are many different types of proofs in geometry that the only ones I can think of from the top of my head is SSS Postulate and SAS Postulate. SSS tells you that if you get a certain number then you can be able to count them as equal. And also the SAS Postulate says that all the sides and angles are equal depending on the number. I hope that answers your question.
• You can split ABC into 2 equal parts with a midpoint E. Then ACE and BCE are equal to x degrees(Remember that the angle of ACB is 2x degrees.) Then you get a right angle ECD. The value of ECD is (x+10)degrees and the value of BCE is x degrees. Since ECD is complementary, you can combine to get 2x+10=90. From here on, x is equal to 40 degrees. Isn't this easier?
• Is this last video of congruency?
• No there are more videos in the section "Theorems Concerning Quadrilayeral Properties."
• My question is an isosceles triangle but it has 3 other numbers on the outside how do i solve for that there are two 7 one 9 and one 80 (the 80 is in the inside) how do i solve for that?
• I am not sure what you are wanting to solve for, but since the video is about angles, it is pretty basic.
IF the 80 is one of the base angles (across from one of the congruent sides), the other base angle is 80 and the third is 180-(80+80)=20 degrees for the vertex angle (between the two congruent sides).
If 80 is the vertex angle, 80+x+x=180, subtract 80 and combine like terms to get 2x=100, divide by 2 to get each base angle is 50 degrees.
In both cases, the principal is that if you have congruent sides, the two angles opposite the congruent sides (called base angles) have to be congruent also.