Alternative proof for gradient of perpendicular lines

I thought the proof m1.m2 = -1 for gradient between perpendicular lines we discussed last week may be too difficult for lower abililty students.

Did some googling for an alternative. We could use simple diagram and rotate the line 90 degrees to demostrate the gradient relationship.



After rotating 90 degrees, from the diagram, the gradient becomes - B/A (vertical rise / horizontal run).
Is this way of proving good enough?

Reference: http://mathforum.org/library/drmath/view/54496.html

Group F