In article <firstname.lastname@example.org>, rb <email@example.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).