cho tam giac ABC vuong tai A ,goc C =30 do, BC = 10 a , tinh AB ,AC b , ke tu A cac duong thang AM ,AN lan luot vuong goc voi cac duong phan giac goc trong va goc ngoai cua goc B . CMR :MN song song BC va MN=BC c , CMR: tam giac MAB dong dang voi tam giac ABC . tim ti so dong dang