Final answer:
In an isosceles triangle that is not equilateral, the term used for the angle between the congruent sides is a vertex angle. The angles opposite the equal sides are called base angles, and all triangles have angles that sum up to 180 degrees.
Step-by-step explanation:
In an isosceles triangle that is not equilateral, the angle between the congruent sides is called a vertex angle. This is because in any triangle, the vertex refers to the point where two sides of the triangle meet, and in the case of an isosceles triangle, the vertex angle is the angle between the two sides that are of equal length. On the other hand, the angles that are opposite to the equal sides are called the base angles, and they are equal to each other due to the properties of isosceles triangles, which state that if two sides are equal, the angles opposite those sides are also equal.
To summarize, within a triangle, especially an isosceles one, there are distinct terms used to describe specific angles and sides. The sides of equal length are referred to as the 'legs', and the third side is known as the 'base'. It's important to note that all triangles, regardless of their type (isosceles, equilateral, or scalene), have internal angles that add up to 180 degrees—a foundational concept in trigonometry and geometry.