Synonym for perpendicular line.

A tangent is perpendicular to a radius drawn to the point of tangency of a circle. The line that contains the radius is the normal to the circle at the point of tangency.

### Example

For the point \(P\left ( x_{1}, y_{1} \right )\) on the circle \(\left ( x-h \right )^{2}+\left ( y-k \right )^{2}=r^{2}\), the slope of the normal is \(\frac{y-k}{x-h}\) and its equation is\(\frac{\left ( y-y_{1} \right )}{\left ( x-x_{1} \right )}=\frac{\left ( h-y_{1} \right )}{\left ( k-x_{1} \right )}\).