It is known that, as shown in the figure, the straight lines a and B are cut by the straight line C, and ∠ 1 + ∠ 2 = 180 degrees. To prove that a is parallel to B, do you know how many methods to prove that a is parallel to B? To solve these problems, we must use the knowledge of grade two (including grade two)

It is known that, as shown in the figure, the straight lines a and B are cut by the straight line C, and ∠ 1 + ∠ 2 = 180 degrees. To prove that a is parallel to B, do you know how many methods to prove that a is parallel to B? To solve these problems, we must use the knowledge of grade two (including grade two)

As shown in the picture,
①∵∠1+∠2=180°,∠2+∠3=180°,
∴∠1=∠3,
‖ a ‖ B (the same position angle, two lines parallel)
 
②∵∠1+∠2=180°,∠2+∠4=180°,
∴∠1=∠4,
‖ a ‖ B (equal internal stagger angle, two parallel lines)
 
③∵∠1+∠2=180°,∠2=∠5,
∴∠1+∠5=180°,
∥ a ∥ B (the inner angles of the same side are complementary, and the two lines are parallel)