Date: Feb 19, 2008 12:27 PM
Author: briggs@encompasserve.org
Subject: Re: Geometry Problem

In article <mn.9c417d82c4b92867.77903@nomail.com>, rb <wnowmailw@nomail.com> writes:
> I.M. Soloveichik wrote:
>> There is not enough information
>
> How do you prove it? ;)
>

>> unless that angle at D which looks like it is
>> 90 is supposed to be 90--in that case the angle x is 40 degrees.

>
> You shouldn't see what "looks like" if you cant say it for sure.
> The picture is only indicative. Drawing it with the right angles could
> be an help but it's a non-necessary condition to solve a problem.


In general, one also needs to beware of what a drawing purports to show
you "for sure".

For instance, if you draw a scalene triangle ABC such that the
angle bisector of angle A intersects the perpendicular bisector
of BC _inside_ the triangle then you're well on your way to proving
that all triangles are isosceles. (One of my all time favorite "proof"s).