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Tough geometry problem about triangles, proof

  1. Jul 1, 2016 #1
    1. The problem statement, all variables and given/known data
    let be ABC a generic triangle, build on each side of the triangle an equilater triangle, proof that the triangle having as vertices the centers of the equilaters triangles is equilater

    2. Relevant equations
    sum of internal angles in a triangle is 180, rules about congruency in triangles

    3. The attempt at a solution
    i know two ways to demontrate that a triangle is equilater, congruency of the sides or same angles, but i have no clue on how to do it, i didn t found any relation between the triangles, only some opposite angles
     
  2. jcsd
  3. Jul 1, 2016 #2

    QuantumQuest

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    If you haven't already done so, create a good diagram. Then see what does the center of the equilateral triangles give you regarding properties and see if it's better to work with angles or sides for the requested triangle.
     
  4. Jul 1, 2016 #3
    i still cant find a way
     
  5. Jul 1, 2016 #4

    QuantumQuest

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    By now, I assume you have a good diagram. Now, have you learned about cevians?
     
  6. Jul 1, 2016 #5
    I didn't get in deep with the argument, I just know the definition of it, a segment that connects a vertex with the opposite side
     
  7. Jul 1, 2016 #6

    berkeman

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    @Cozma Alex -- please post your sketch of the problem or your thread will be deleted.
     
  8. Jul 1, 2016 #7

    jedishrfu

    Staff: Mentor

    As a suggestion, if you have a smartphone with a camera, you can hand draw a clean well labeled diagram, take a picture and upload the image to this thread.
     
  9. Jul 2, 2016 #8
    I got the image of the problem
     

    Attached Files:

  10. Jul 2, 2016 #9
    Since the triangle ABC in the given image of the problem doesn't look generic, and following yours advice I also drawn one by myself

    (Sorry for the double posting of the image, it was because of the lag)

    I think a way is to use similiar triangles and then get to "same angles way"
     

    Attached Files:

    Last edited: Jul 2, 2016
  11. Jul 2, 2016 #10

    QuantumQuest

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    Do you know which theorem is this thing you're trying to prove?
     
  12. Jul 2, 2016 #11
    Similar triangles leads to equals angles between those triangles
     
  13. Jul 2, 2016 #12

    QuantumQuest

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    It has a specific name. Anyway, have you learned / studied the properties of equilateral triangles? If not take a look here https://en.wikipedia.org/wiki/Equilateral_triangle. This is absolutely necessary. Second, do you know the fundamentals of trigonometry ( including law of sines and cosines)? I am asking this, because my hints so far, go towards a geometric direction of the proof involving a little fundamental trigonometry.
     
  14. Jul 2, 2016 #13
    Yeah I know trig
     
  15. Jul 2, 2016 #14

    QuantumQuest

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    OK, so I am assuming that you know well both things I pointed out in my previous post. Now, going to your sketch, can you see any way to relate a side of the inner triangle (the one you want to prove equilateral) with some known angles from the outer equilateral triangles? As an extra hint, look at cevians which are among other things, angle bisectors.
     
  16. Jul 2, 2016 #15
    I see that every side is opposite to an angle: 60+ angle of the ABC

    Side opposite to angle reminds me sine theorem but that works only in the same triangle and those are different
     
  17. Jul 2, 2016 #16

    QuantumQuest

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    So, you want to find a triangle formed by one side of the inner triangle and two cevians of the outer triangles, in order to have one triangle to work on. Also, what about cosines law as well?
     
  18. Jul 2, 2016 #17
    I can find the lenght of AQ and AS in function of AC and AB and with cosine law find the lenght of a side of the inner triangle using the angle between AQ and AS is that the way?
     
  19. Jul 2, 2016 #18

    QuantumQuest

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    Yes. Form the law of cosines and see how you can find the length for each side included in the expression, in order to substitute.
     
  20. Jul 2, 2016 #19
    Ok thanks for your help and patience :)
     
  21. Jul 2, 2016 #20

    QuantumQuest

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    You 're welcome. Now, can you see how to proceed further? There are some more steps to do in order to be done.
     
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