Now use the information you already have about finding the equation of a line and apply it to the dashed lines shown in these triangles. Remember, to find the equation of a line you need a point ...

If lines are perpendicular to each other then \({m_1} \times {m_2} = - 1\) conversely if \({m_1} \times {m_2} = - 1\) then the lines are perpendicular to each other.