Examples of eight-character model in junior high school mathematics are difficult

Example 6: Parallel lines passing through point C and point B, point AB and point AC respectively intersect at point F. If DF is connected, the quadrilateral ABFC is a parallelogram.

∵AD is the center line on the side of BC, ∴AD and BC are the diagonal lines of parallelogram ABFC.

In △ABF, AB+AF must be greater than AF, that is, AB+AC is greater than AF.

If both sides of the inequality sign are divided by 2 at the same time, this problem can be proved.

Example 5: Let A be greater than C and greater than B, and make ∠CAB form an acute angle.