There are three main ways to prove that a triangle is isosceles:
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Measure two sides of the triangle and check if they are the same length. An isosceles triangle has at least two sides that are congruent (the same length).
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Check the angles opposite the two sides. In an isosceles triangle, the angles opposite the two congruent sides will be congruent as well. So measure the angles and see if any two are the same measure.
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Use properties of isosceles triangles. We know that in an isosceles triangle, the angle bisector of the base angles will bisect the base. So construct an angle bisector and see if it divides the base exactly in half.
If any one of these conditions is true, then the triangle is isosceles. Let me know if you need any clarification or have additional questions!
Related tutorial videos:
Geometry Proofs - Isosceles Triangles - SAS & AAS
Show that this triangle is Isosceles (OR equilateral OR scalene)
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