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.

To find the area of a right angled triangle, you need to multiply the two perpendicular sides together and halve the result. On occasion, you may need to find the area of more irregular shapes ...