Calculus 8th Edition

a) Formula to calculate the equation of tangent plane for the the surface $f(x,y)=z$ at the point $(x_1,y_1,z_1)$ is: $z-z_1=f_x(x_1,y_1)(x-x_1)+f_y(x_1,y_1)(y-y_1)$ b) Formula to calculate the equation of tangent plane to the level surface $F(x,y,z)=k$ at the point $(x_1,y_1,z_1)$ is: $F_x(x_1,y_1,z_1)(x-x_1)+f_y(x_1,y_1,z_1)(y-y_1)+f_z(x_1,y_1,z_1)(z-z_1)=0$