Planar Orthogonality

Suppose we have two lines, \(y_1\) and \(y_2\), defined by,

\[ y_1 \equiv m_1 x_1 + b_1\,\text{ and,} \] \[ y_2 \equiv m_2 x_2 + b_2\]

If the two lines are orthogonal, then,

\[ m_1 \cdot m_2 = -1\]