Figure 31 
\(f_{y} (x_{0} ,y_{0} )\) is the slope of the tangent line to the curve \(z=f(x_0,y)\) at \((x_{0}, y_{0}, z_{0})\).
\(f_{y} (x_{0} ,y_{0} )\) is the slope of the tangent line to the curve \(z=f(x_0,y)\) at \((x_{0}, y_{0}, z_{0})\). \(f_{y} (x_{0} ,y_{0} )\) is the slope of the tangent line to the curve \(z=f(x_0,y)\) at \((x_{0}, y_{0}, z_{0})\).