How can you prove triangles are identical using only their side lengths?

Proving the SSS triangle congruence criterion using transformations | Geometry | Khan Academy · 5:46 · Video: Khan Academy · Watch at the source

In short: The side-side-side congruence criterion is a fundamental geometric rule. This video demonstrates how to prove it using the rigid transformation definition of congruence, showing why matching side lengths guarantee identical triangles.

In geometry, the side-side-side (SSS) criterion states that if three sides of one triangle are equal to three sides of another, the triangles are congruent. This video explores the formal proof of this concept by applying rigid transformations—geometric movements like rotations, reflections, or translations that preserve shape and size.

By using the definition of congruence based on these transformations, the proof confirms that if the sides match, the triangles must be identical in form. This approach moves beyond simple memorization, grounding the SSS rule in the underlying mechanics of geometric space.

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