Side opposite the vertex angle is the base.

So, ∠B ≅ ∠C, since corresponding parts of congruent triangles are also congruent. Based on this, △ADB ≅ △ADC by the Side-Side-Side theorem for congruent triangles since BD ≅ CD, AB ≅ AC, and AD ≅ AD. Using the Pythagorean Theorem where l is the length of the legs. ABC can be divided into two congruent triangles by drawing line segment AD, which is also the height of triangle ABC. Refer to triangle ABC below.ĪB ≅ AC so triangle ABC is isosceles. The base angles of an isosceles triangle are the same in measure.

The figure below shows these parts of an isosceles triangle. The altitude from the base of an isosceles triangle to its opposite vertex divides the triangle into two congruent right triangles. Altitude - the perpendicular distance from the vertex of a triangle to the opposite side.Base angle - the angles adjacent to the base of the isosceles triangle these are the two congruent angles.Vertex angle - the angle opposite the base of the isosceles triangle.Base - the third side of the triangle that is not congruent to the other two.Legs - the congruent sides of the triangle.The parts of an isosceles triangle are its legs, base, vertex angle, base angle, and altitudes. The yellow part of the pizza forms an isosceles triangle, as shown by the side and angle markings. A real life example of an isosceles triangle is a slice of pizza given that we exclude the curved part of the pizza crust, as shown in the figure below. What does an isosceles triangle look likeĪn isosceles triangle can look like various different things depending on the type of isosceles triangle. To identify if a triangle is isosceles, check whether it has two congruent sides and angles if it does, it is isosceles if it doesn't, it is not isosceles. The isosceles triangle definition is a triangle that has two congruent sides and angles. The tally marks on the sides of the triangle indicate the congruence (or lack thereof) of the sides while the arcs indicate the congruence of the angles.

The figure below shows an isosceles triangle example. Since the sides of a triangle correspond to its angles, this means that isosceles triangles also have two angles of equal measure. Home / geometry / triangle / isosceles triangle Isosceles triangleĪn isosceles triangle is a triangle that has at least two sides of equal length.

Right isosceles triangle definition