1. It is given than B is the midpoint of AC. 2. By the definition of midpoint, AB and BC are congruent. 3. It is given that the midpoint forms a right angle. 4. Thus, by SAS, the two triangles are congruent. 5. When two triangles are congruent, opposite sides are congruent. 6. Thus, it follows that AD is congruent to DC. 7. When two legs are congruent, the triangle is isosceles. 8. Thus, ADC is isosceles.