Why is the product of the slopes of perpendicular lines always negative?

Answer 1

Yes the product will be negative, in fact the product will equal negative one (-1).

Think about this. Suppose you have a line y = mx + b, and you want to rotate it 90° counterclockwise. This new line would be like in a coordinate system (x',y') [the x' and y' are called x-prime and y-prime, and differentiate between the original and new coordinate system], where the x' axis runs along the y axis in the positive y direction, and the y' axis runs along the x axis in the negative x direction. So the new line y' = mx' + b with x' = y and y' = -x, is: -x = my + b. Solving for y in the new equation gives y = (-1/m)x - (b/m). So the new slope (-1/m) times the original slope (m) equals (-m/m) = -1, as long as the original slope was not zero.