Among the eight angles formed by two straight lines being cut by the third straight line, there are four pairs of congruent angles, two pairs of internal staggered angles and two pairs of internal angles on the same side. As shown in the figure 1, ∠ 1 and ∠5 are ipsilateral internal angles, ∠2 and ∠6? It's the same inner angle.
The properties of the ipsilateral internal angle are as follows:
Determination of parallel lines: the internal angles on the same side are complementary and the two straight lines are parallel.
The nature of parallel lines: two straight lines are parallel and complementary to each other's internal angles.
2. Parallel lines are equally divided into segments: if a group of parallel lines have equal segments on a straight line, then the segments on other straight lines are also equal. As shown in figure 2, if l1/L2//L3 and AB = BC, then a1b1= b1?
The bisection of parallel lines is the basis of proving the median line theorem of triangle and trapezoid, and it is also the basis of the "proportion theorem of parallel lines".