cho tam giac abc(ac>ab)ad la duong phan giac trong. Qua c ke tia cx sao cho cb nam giua ca la e. Duong thang di qua d song song voi ce cat be o i.a)CM tam giac adb dong dang tam giac cde va ab/ac=bi/ie. b) CM tam giac bde dong dang tam giac adc. c)CM ab•ac-bd•dc=ac^2