If you have two parallel lines, then they have the same slope. If you have another line, in the same plane as the first two lines, and it is not parallel to them, then the angle of intersection with the first line will be the same as the angle of intersection with the second line. If this angle is 90°, then the third line is perpendicular to the first line and also to the second line.
For an example, think of a rectangle. Two opposite sides are parallel. Now pick one of the 3rd or 4th sides. Each of these sides is perpendicular to both the first side and the second side.
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the parallel lines never intersect each other but they both intersect the line they are perpendicular to
They are parallel to each other.
Parallel lines run in the same direction. If they are intersected by one line and that line is perpendicular to both, then it proves that the lines run parallel to each other.
They're parallel.
true