The definition of the derivative allows us to define a tangent line precisely. {{CMD_156}} Let {{MATH_157}} be a function differentiable at {{MATH_158}}. The {{CMD_159}} {{MATH_160}} at the point {{MATH_161}} is the line that passes through the point {{MATH_162}} whose slope is equal to {{MATH_163}}. {{CMD_164}} Using the point-slope equation for a line, the tangent line is given by the equation {{MATH_165}}