Tangent line
The line that touches a curve at a point and has the same slope as the curve there.
At a point (a, f(a)) on a differentiable curve, the tangent line has equation y = f(a) + f'(a)(x − a). It is the best linear approximation to the curve near that point — close to (a, f(a)) the curve and the line are nearly indistinguishable.
Formula
y = f(a) + f'(a)(x − a)