Tangent lines are straight lines that pass through a given curve and have the slope of the curve at the point where they intersect. The idea of tangent lines can be extended to higher dimensions in the form of tangent planes and tangent hyperplanes. A normal line is a line that is perpendicular to the tangent line or tangent plane. Wolfram|Alpha can help easily find the equations of tangents and normals to a curve or a surface.
Find a tangent line to a curve.
Find the tangent to the graph of a function at a point: