How To Prove That A Triangle Is Isosceles?

There are a few different ways to prove that a triangle is isosceles:

• Measure two sides of the triangle and show that they are the same length. An isosceles triangle has at least two sides that are congruent.
• Use a protractor to measure two of the triangle's angles and show that they are congruent. The angles opposite the equal sides of an isosceles triangle are also congruent.
• Apply the angle-side-angle criterion for triangle congruence. Show that two angles and the included side are the same for the given triangle and a known isosceles triangle. This proves the triangles are congruent so the given triangle must be isosceles.

So in summary, to prove a triangle is isosceles you need to demonstrate either two congruent sides, two congruent angles, or angle-side-angle congruence to a known isosceles triangle.

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Related tutorial videos:

Geometry Proofs - Isosceles Triangles - SAS & AAS

Show that this triangle is Isosceles (OR equilateral OR scalene)