...

... AB,BC,BD ... ...

http://img198.imageshack.us/img198/5589/55943154.png

E EB EC BEC

EC F E B BFE

..

BEF

BFE

FBE 180-BEF-BFE

EF

BFE BF,BE SINE RULE

COSINE RULE BFC,CBE BC FC ( )....