SSA and Non-congruent TrianglesDate: 08/13/98 at 13:23:24 From: Carolyn White Subject: Congruence of triangles I'm trying to stay ahead of my teenage daughter, but it's been 35 some years since high school geometry. I'm using Barron's "Geometry The Easy Way," and things are coming back into focus. But I can't visualize why you cannot necessarily conclude that two triangles are congruent when "two sides and an angle that is not included of one triangle are congruent to the corresponding parts of the other triangle" (page 97). I know SAS proves congruence, but it seems to me that if one set of angles is congruent and two sets of sides are congruent, you have enough to establish congruency of the two triangles by SSA. What am I overlooking? Could you provide an example to illustrate non-congruent triangles that meet the criteria? I'd appreciate some guidance. Thanks! Date: 08/13/98 at 13:56:20 From: Doctor Barrus Subject: Re: Congruence of triangles Hi, Carolyn! In some SPECIAL cases (which depend on the lengths of the sides and the size of the angle involved), SSA is enough to show congruence. However, it's not always enough. Consider the following triangles: . A . . . . ............. B C D . . . . . ....... E F Here: side AB is congruent to side DE (S) side AC is congruent to side DF (S) angle C is congruent to angle F (A) But the triangles are not congruent, as you can see. What happens is this: if you draw a vertical line through point A in the first triangle, you can sort of "flip" side AB around this line to get the second triangle. If we were to lay one triangle on top of the other and draw the vertical line, this is how it'd look. A,D | /|\ . / | \ . /..|..\...... B | E C,F You can see that side DE is just side AB flipped around the line. So we haven't changed the length of the side, and the other side AC (or DF) is unchanged, as is angle C (or F). So these two triangles that have the same SSA information, but they're not congruent. I hope that makes sense. If not, feel free to write back. Good luck! - Doctor Barrus, The Math Forum Check out our web site! http://mathforum.org/dr.math/ |
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