Slope of Normal is Minus Reciprocal of Tangent
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Theorem
Let $C$ be a curve defined by a real function which is differentiable.
Let $P$ be a point on $C$.
Let the curvature of $C$ at $P$ be non-zero.
Let $r$ be the slope of the tangent to $C$ at $P$.
Let $s$ be the slope of the normal to $C$ at $P$.
Then:
- $r = -\dfrac 1 s$
Proof
By definition, the normal to $C$ at $P$ is defined as being perpendicular to the tangent at $P$ and in the same plane as $P$.
The result follows from Condition for Straight Lines in Plane to be Perpendicular.
$\blacksquare$