Slope, m = tan θ =( y2-y1)/( x2-x1)
If the line l1 is parallel to l2 then their inclinations are equal, α = β à tan α = tan β à m1 = m2
If the lines l1 and l2 are perpendicular then β = α + 90 °. or tan β = tan( α + 90 °) = – cot α =-1/ tanα or m2 =-1/ m1 or m1 * m2 = – 1