It is proved that the sum of the internal angles of a triangle is equal to 180.

Feeling: In △abc, ∠a, ∠b and ∠c are three internal angles. To prove ∠a+∠b+∠c= 180, that is to say ∠A+∞. Using the characteristics of parallel lines, it is necessary to draw a parallel line through point A to achieve the goal.

A is ef BC.

∴

∠b=∠ 1，∠ C = ∠ 2。 (Two straight lines are parallel with equal internal angles).

∵

∠ 1+∠bac+∠2= 180，

∴

∠ B+∠ BAC+∠ C = 180。 (Equivalent substitution)

Can't draw.