过D做AB、AC垂线,交AB于E、AC于FDE=DF=X (角平分线)3X+4X=12(面积和) X=12/7AE=DE(45度)BE=3- 12/7=9/7BD^2=DE^2+BE^2=144/49+81/49=225/49BD=15/7
BD等于2.5
CB=5