题目

If you think that running marathons will help you live a long and healthy life, new research may come as a shock. According to a recent scientific study, people who do a very strenuous workout are as likely to die as people who do no exercise at all.    Scientists in Denmark have been studying over 1，000 joggers (慢跑者)and non-joggers for 12 years. The death rates from the sample group indicate that people who jog at a moderate pace two or three times a week for less than two and a half hours in total are least likely to die. The best speed to jog at was found to be about 5 m已知AB=AD，BC=DC．求证：AC平分∠BAD．