In this section, we consider the problem of finding the tangent plane to a surface, which is analogous to finding the equation of a tangent line to a curve when the curve is defined by the graph of a function of one variable, [latex]y=f(x)[/latex]. The slope of the tangent line at the point [latex]x=a[/latex] is given by [latex]m=f^{\prime}(a)[/latex]; what is the slope of a tangent plane? We learned about the equation of a plane in Equations of Lines and Planes in Space; in this section, we see how it can be applied to the problem at hand.