In an isosceles triangle ABC. Angles A and B are 56 degrees. Which side of the triangle is its base?

Before answering the question of which of the sides of the isosceles triangle ABC indicated in the task is its base, one should recall one of the properties of an isosceles triangle.
This property of an isosceles triangle is as follows: if any triangle is isosceles, then the angles of this triangle at its base are equal.
By condition, it is given that the angles A and B of the considered isosceles triangle ABC are equal to 56 °.
Thus, the base of a given isosceles triangle ABC is side AB, since the angles at a given base AB have the same magnitude.