Let L1 and L2 be lines defined by
Show that L1and L2 are parallel if and only if u = kv for some k R
u & v are the direction ratios of the lines L1 & L2
for the lines to be parallel the direction ratio of one line must the scalar multiple of the other line.
so if u = kv i.e u is the multiple of v hence the line L1 & L2 are parallel to each other
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