isosceles triangle

In an isosceles triangle, two of the three sides of the triangle have the same length.

The triangle to the right is an isosceles triangle, even though it may be hard to tell.

Scroll down for sure-fire ways to tell if a triangle is isosceles or not.

 


In the above triangle, the red dashed line is called an altitude. It is the perpendicualr line from the midpoint of the base to the opposite vertex. It forms two right triangles.

If we can show that these two right trianglesare congruent, then we can prove that the above triangle is indeed an isosceles triangle.

We know that the altitude bisects the base so now we have two sides of the two right triangles are congruent. Since the right angles are in between these two sides we can see that by side-angle-side, we have two congruent triangles. Thus the two hypotneuses are congruent so we indeed have an isosceles triangle.

Notice that the two base angles are also congruent. This is a property of isosceles triangles.
Click here for a GSP script to form an isoscoles triangle given an altitude, then a base.

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