1. The problem statement, all variables and given/known data Proof that if the slopes of two lines a1, a2 (that are not vertical), m1,m2 satisfy: m1*m2 = -1, then the lines are perpendicular. 2. Relevant equations 3. The attempt at a solution I tried to use the tan function, so that m1 = tanΘ where Θ1 is the angle of the line formed from x axis. and m2 = tanΘ2. now tanΘ2*Θ1 = -1, I tried using the tan(Θ1+Θ2) = but after simplifing i got (tanΘ1 - tankΘ2)/2 not sure if im on the right way, rather solve it using trig. ok found the answer, i had to use the points (1,m1), (1, m2), form a triangle and check for Pythagorean theorem.