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).