Exercise 1 Page 315

Consider $△ABD.$ Notice that all of the sides are marked to indicate that they are congruent segments.

When a triangle has three equal side lengths, it not only tells us that it is an equilateral triangle but also that the angle measures are equal. Since this triangle has three congruent side lengths and therefore three congruent angles, we know that $△ABD$ is an equiangular triangle.