关于一道是学提

已知四边形ABCD是由一个等边三角形和一个顶角为120°的等腰三角形拼成的，将一个含60°角的纸片的顶点放在D处，将该纸片旋转，设该纸片60°角的两边分别交直线AB,AC于M,N两点，当M,N分别在AB,AC上时求证BM+CN=MN

Raw material: 250 overcomes the pig tenderloin , the egg (makes an appointment with one and a half) 75 grams, 30 overcomes with no result careful 50 grams of bean flour , 5 grams of refined salt , 5 grams of cooking wine , 5 grams of soy sauce , vinegar , 40 grams of white sugar , 5 grams of ginger rice , 5 grams of garlic rice , 20 grams of chopped green onion , 1 gram of gourmet powder , 5 grams of sesame oil , 20 grams of water bean flour, are 200 grams a delicious soup, rape oil (consumes 100 grams) 1000 grams. Method for production: 1, cuts fresh clean pig tenderloin into 1 centimetre thick movie , the straight knife horizontal stroke tendon seizes one time by force , behead able 4 centimetres strong point , 1 centimetre broad strip , put the bowl inner in, use salt , expect sprinkling code taste dipping, the egg and with no result thin bean flour blend fully grown egg thick fluid , soy sauce , salt , vinegar , white sugar , gourmet powder , water bean flour , delicious soup add ready-made sauce made of the seed powder of Gorgon euryale or starch juice. 2, on stir-frying a boiler purchasing scorching fire, readjust oneself to a certain extent oil fever of having a temperature till fifty percent, flesh is mixed evenly with egg thick fluid putting the frying pan inner inside exploding till maturity fish up, after the oil temperature rises again till seventy percent, introduction meat explodes till soft incense of surface and fishes up when assuming golden yellow, strain to get rid of oil putting the set inner in. The boiler washes 3, fever of clean , readjusting oneself to a certain extent oil to have a temperature till forty percent , put in the ginger , garlic , onion summary stir-frying several time , cook to enter sauce made of the seed powder of Gorgon euryal