这个题目不用余弦定理因为BD‖CE，CE平分∠ACB∴∠CBD=∠D∴CD=CB=10∵AB/AD = 12/(AC+CD)=12/(AC+CB)=12/18 = 2/3AC/AB=8/12=2/3∴AB/AD=AC/AB又∵∠A=∠A∴△ABC∽△ADB∴BC/BD=2/310/BD=2/3∴BD=15
