Our approximation of the isoceles triangle seems to suggest that the distance from point B to the intersection of the dashed line is equal to the radius of c2. Therefore we need to copy that distance. This can be done easily by using point B as the center of a circle with a radius equal to the radius of c2. These two circles will be congruent.
When we connect these two points to help form our isoceles triangle, what part of the triangle will the dashed line become? Can you apply a name to it? Can you state its definition?