Triangle ProofDate: Sat, 18 Feb 1995 22:01:29 -0800 (PST) From: Ralph Sierra Subject: Help, please! I'm a student in high school and I need help on this one problem. I'm in geometry and stumped. I'm thinking that maybe to figure it out, it needs to be said that if 2 sides of a triangle are not congruent, then the angles opposite them are not congruent, and the larger angle is opposite the longer side. But I'm not sure how to say it in a proof. I have an idea but am getting a little confused. If you could help me, I would appreciate it. A /|\ / | \ / | \ / | \ / | \ / | \ / | \ / | \ / | \ /_________|_________\ B D C B, D and C are points of the line L such that B-D-C and BD<DC. If segment AD is perpendicular to L, prove that AB<AC. Thank you, - Michelle Date: Sun, 19 Feb 1995 02:16:52 -0500 (EST) From: Dr. Ken Subject: Re: Help, please! This is a job for the law of Sines. As you may know, if A, B, and C are the angles of a triangle and a, b, and c are the sides opposite them, then a b c ------ = ------ = ------ Sin[A] Sin[B] Sin[C]. So in your case, if what you need to show is that AB<AC, a good way to do that would be to show that angle C is smaller than angle B. Then you'd be able to say that AB AC -------- = -------- Sin[C] Sin[B], And since Sin[C] < Sin[B], AB<AC. Does this help you? If you buy that, then all you have to do is show that angle C is smaller than angle B. I hope this train of thought gets you somewhere, and write back if you're still stumped. -Ken "Dr." Math Date: Sun, 19 Feb 1995 08:50:45 -0800 (PST) From: Ralph Sierra Subject: Re: Help, please! In our class we have not studied the law of sines yet, so I'm not following your directions. I'm sure you're correct, but can you please explain it a little simpler? Thanks - Michelle Date: Sun, 19 Feb 1995 12:57:34 -0500 (EST) From: Dr. Ken Subject: Re: Help, please! Hello there. Not allowed to use the law of sines, huh? Well, perhaps we can do this with the Pythagorean theorem. Oh, wow, in fact it's easiest this way. We know that BD<CD. Using the Pythagorean theorem (BD^2 + AD^2 = AB^2 and CD^2 + AD^2 = AC^2), what can you conclude about the lengths of AC and AB? I think this is an easier way than the way I was trying to explain before. -Ken "Dr." Math |
Search the Dr. Math Library: |
[Privacy Policy] [Terms of Use]
Ask Dr. Math^{TM}
© 1994- The Math Forum at NCTM. All rights reserved.
http://mathforum.org/dr.math/