When a doctor tries to diagnose a patient, he or she looks for evidence to try to reach a conclusion.  They may run tests or ask a patient about his or her symptoms.  How can a doctor know for sure that a bone is broken?  They may take an x-ray to prove that the bone has a fracture.  Doctors apply a reasoning process to determine a final diagnosis.  Though they do not take x-rays, mathematicians use similar types of reasoning to prove things in geometry. In this unit, you will learn more about the two types of reasoning and the process by which mathematicians prove that things are true.