|Manipulative 1: SAS Congruence. Created with GeoGebra.|
Two triangles are congruent if two adjacent sides and the angle contained by the sides are congruent with corresponding sides and angle of the other triangle. In this case we say that the triangles are SAS congruent. SAS stands for side, angle, side.
|1||Start with triangle ABC and DEF. Let AB be congruent to DE. Let BC be congruent to EF. Let ∠ABC be congruent to ∠DEF.||Starting conditions.|
|2||The claim is that AC ≅ DF, ∠BCA ≅ ∠EFD, ∠CAB ≅ ∠FDE, and ΔABC ≅ ΔDEF.||Claim|
|3||If ΔABC is placed on top of ΔDEF, the point A is on top of point D. Also, the line segment AB is on top of DE, and the line segment BC is on top of DF. Since point A coincides with D and point C coincides with point F, then the line segment AC must coincide with the line segment DF.||Euclid Elements Book 1 Common Notion 4: Things which coincide one with another are equal.|
|4||Since all sides of ΔABC coincide with all sides of ΔDEF, ΔABC ≅ ΔDEF. It must also be true that ∠BCA ≅ ∠EFD and ∠CAB ≅ ∠FDE. QED.||Euclid Elements Book 1 Common Notion 4: Things which coincide one with another are equal.|
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