Triangles can only be congruent if you can prove that they have one of these three properties: 1. All the sides are the same lengths as the sides on the other triangle (e.g. both have sides of 3, 4 and 5 cm) 2. Two of the sides, and the angle between them are the same in both triangles 3. Two of the angles and the corresponding side to them (the side that is attached to both corners where the angles are measured) are the same in both triangles. If any of the above can be proved to be true then the triangles are congruent. However, if any one of the conditions above are proved to be false - for example if one triangle has two sides the same, but one has the angle between them of 40 degress and the other at 41 degrees, (breaking rule 2) then the triangles are not congruent.
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sssThere are five methods for proving the congruence of triangles. In SSS, you prove that all three sides of two triangles are congruent to each other. In SAS, if two sides of the triangles and the angle between them are congruent, then the triangles are congruent. In ASA, if two angles of the triangles and the side between them are congruent, then the triangles are congruent. In AAS, if two angles and one of the non-included sides of two triangles are congruent, then the triangles are congruent. In HL, which only applies to right triangles, if the hypotenuse and one leg of the two triangles are congruent, then the triangles are congruent.
If triangles have the corresponding sides congruent then they are congruent. SSS If two triangles have two sides and an included angle congruent then they are congruent. SAS If two triangles have two angles and an included side congruent then they are congruent. ASA SSA doesn't work.
Once you have shown that two triangles are congruent you can use CPCTC (corresponding parts of congruent triangles are congruent) to show the congruence of the remaining sides and angles.
The term for two triangles that are congruent after a dilation is similar.
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