# Tangent approximation

The tangent approximation method is a method in Calculus employed to find the equation of a line tangent to the curve. One must know the slope of the curve and a point on the curve. The slope is usually found by taking the derivative of the equation and equating it to the change in y over the change in x:

$\frac{dy}{dx}\ = \frac{rise}{run}\ = \frac{y - y'}{x - x'}\$

Utilizing cross-multiplication, this yields:

$y - y' = \frac{dy}{dx}\ x - x'$

When ( x' , y' ) is a known point on the line.