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