The last topic that we have to discuss in this section is that of parallel & perpendicular lines. Following is a sketch of parallel and perpendicular lines.

Suppose that the slope of Line 1 is m₁ and the slope of Line 2 is m₂.  We can relate the slopes of parallel lines & we can relate slopes of perpendicular lines as.

parallel : m₁ = m₂

perpendicular : m₁m₂  = -1  or m₂= -1/m₁

Note down that for perpendicular lines there are two forms of the equation. The second is the morecommon & in this case usually we say that m₂ is the negative reciprocal of m₁.