clarification (1 Viewer)

[Real Madrid]

I have to prove that the lines are perpendicular but according to m1m2=-1, it doesn't show

 

[Trebla]

A geometric proof should suffice. The diagram shows an isosceles triangle which is bisected by y = 5. Remember that the bisector from the 'non-equal' apex angle, cuts the base at right angles. No need for gradients...