證明:AB+AC大於DB+DC(D為三角形ABC的一個內點)

Sorry i dont have the equation on hand, so i can only tell you how to do:

Draw a triangle ABC, and add D inside in any position,
draw line AD, CD & BD.
So you can see 3 triangle now.
Then use sine & cosine rule to draw equation, then use subsitution to prove. Also use CAST > 0 to fit into an inequality.

Hope you understand.