Final answer:
The common endpoint of the congruent sides in an Isosceles triangle is called the vertex angle.
Step-by-step explanation:
The angle called that is the common endpoint of the congruent sides in an Isosceles triangle is known as the vertex angle. When thinking of a triangle, we consider a three-sided figure lying on a plane with three angles that add up to 180 degrees.
In an Isosceles triangle, two sides are of the same length (the congruent sides), and the angles opposite these sides are also congruent. The angle formed by these congruent sides is unique and hence called the vertex angle of the Isosceles triangle.
The angle that is the common endpoint of the congruent sides in an Isosceles triangle is called the vertex angle.
Example:
In an Isosceles triangle ABC, if AB = AC, then the angle at vertex B (angle ABC) is the vertex angle.