# Thread: Coordinate Proof... Perpendicular Lines

1. ## Coordinate Proof... Perpendicular Lines

Prove: If the product of the slopes of two lines is -1, then the lines are perpendicular.

I have already proven the converse of this statement. I have no idea where to go / what to do for this proof...

2. If θ1 and θ2 are the angles made by the two straight lines with x-axis, then the angle between the two straight line is given by
θ= θ2 - θ1.
Τhen tanθ = tan(θ2 - θ1) = (tanθ2 - tanθ1)/( 1 +tanθ1*tanθ2) = (m2 - m1)/(1 + m1*m2)
If m1*m2 = -1, tanθ becomes infiifnty which is possible when θ = π/2.