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What is the SAS Congruence Postulate

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SAS (Side‑Angle‑Side) Congruence Postulate:

If two triangles have two pairs of corresponding sides equal in length and the included angles (the angles formed by those pairs of sides) equal in measure, then the two triangles are congruent — i.e., all corresponding sides and angles are equal.

How to use it:
1. Identify two sides in one triangle and the angle between them.
2. Find two corresponding sides in the other triangle and the angle between them.
3. If the two side lengths match respectively and the included angles match, you can conclude the triangles are congruent.

Brief reason (proof sketch):
Fix one triangle, and place the second so one matching side and the included angle coincide. Because the other side of the second triangle has the same length and the same direction from the vertex (given by the equal included angle), the remaining vertex must coincide with the first triangle’s corresponding vertex. Thus the triangles are identical (congruent).

Note: SAS is called a postulate in many elementary geometry texts (an accepted rule). In more formal axiomatic systems it may be proved as a theorem from those axioms.