Prove the theorem that the sum of two sides of a triangle is greater than the third side.

If A and B are on the same side of L, let A be the symmetrical point A 1 about L and connect A 1B, then the intersection of straight line A 1B and L is P.

(Prove AP=A 1P by mirror theorem, and then prove it as above)

(2) If A and B are on the same side, connect A and B and extend L to Q..

(Prove the theorem that the difference between two sides is less than the third side)

If A and B are on different sides, then make A's symmetrical point A2 about L and connect A2B, then the intersection of straight line A2B and L is Q.

(Prove AQ=A2Q with the mirror theorem, and then prove each other in the same way. )