Results 1 to 4 of 4

Math Help - I think I have really messed this proof up! Please help!

  1. #1
    Newbie
    Joined
    Jan 2006
    Posts
    11

    I think I have really messed this proof up! Please help!

    My assignment was simple: prove that a triangle whose base angles are the same is an isosceles. But then I found out I cannot use the base angle theorem.

    Now I found out that I have included unecessary info or else left out a step, my instructor didn't say for sure.

    Does any one have any ideas on an easy way to fix my proof/diagram? Or do you think I should just start over trying to prove it from a different way.

    My diagram and proof are attached.

    Thank you!
    Attached Files Attached Files
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Forum Admin topsquark's Avatar
    Joined
    Jan 2006
    From
    Wellsville, NY
    Posts
    9,671
    Thanks
    299
    Awards
    1
    Quote Originally Posted by e-mom
    My assignment was simple: prove that a triangle whose base angles are the same is an isosceles. But then I found out I cannot use the base angle theorem.

    Now I found out that I have included unecessary info or else left out a step, my instructor didn't say for sure.

    Does any one have any ideas on an easy way to fix my proof/diagram? Or do you think I should just start over trying to prove it from a different way.

    My diagram and proof are attached.

    Thank you!
    First, why this didn't work.
    1) You proved that the wrong pair of sides were equal. (We need AB=AC, you had AB=BC).
    2) The reason this failed was the assumption that the perpendicular bisector of AC intersects the point B. This is not true in general. (Draw a more lopsided triangle and it should be clear.)

    I would suggest that you bisected the wrong thing. I would start over with your triangle, bisect the angle A, and extend the bisector to intersect the line BC at point D. (Note: We canNOT assume BD=CD here. We would need to prove that.) The goal would then to be to prove that triangle ADB is congruent with triangle ADC. (Hint: Look for an angle-side-angle proof.)

    -Dan
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Global Moderator

    Joined
    Nov 2005
    From
    New York City
    Posts
    10,616
    Thanks
    9
    Prove it like this.
    From the vertex draw an angle bisector. Now you have two triangles by a.a.s. and the proof it complete.
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Newbie
    Joined
    Jan 2006
    Posts
    11

    Thanks!

    Thank you Perfect Hacker and Topsquark!

    I will apply what you said as I refigure my crazy triangle!
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Replies: 15
    Last Post: June 8th 2011, 11:13 AM
  2. Replies: 5
    Last Post: October 19th 2010, 10:50 AM
  3. Replies: 0
    Last Post: June 29th 2010, 08:48 AM
  4. Simple but messed up
    Posted in the Calculus Forum
    Replies: 4
    Last Post: January 15th 2010, 02:32 AM
  5. proof that the proof that .999_ = 1 is not a proof (version)
    Posted in the Advanced Applied Math Forum
    Replies: 4
    Last Post: April 14th 2008, 04:07 PM

Search Tags


/mathhelpforum @mathhelpforum