Question

Why two parallel lines have identical directional cosines?

Answer 1

Hey Mate,

Here Is Your Answer

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If the given line is reversed, then the direction cosines will be cos (π − α), cos (π − β), cos (π − γ) or − cos α, − cos β, − cos γ. Thus, a line can have two sets of dc's according to its direction. The direction cosines of two parallel lines are always the same.

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For Diagram,See The Upper Image.
Hope It Helps You.