As the lines are parallel, the distance between them never changes. Therefore whatever point you pick on either lane, the other line will always be the same distance away from it. A picture below should demonstrate it.
In the figure, P and Q are any points on the lines. The right triangles are congruent by AAS. The corresponding congruent sides of the triangles show that the same distance is always obtained between the two lines. ;
You can choose any point on either line when calculating the distance between two parallel lines because the distance remains constant across their lengths. This is due to the nature of parallel lines, which are always equidistant. The distance can be calculated easily from any selected point using their equations.
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