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The Tangent Plane

DEFINITION

The tangent plane of a smooth function z = f(x, y) at (a, b) is the plane with the equation

z - f(a, b) = fx(a, b)(x - a)+ fy(a, b)(y - b).

If we set x = a and y = b in this equation we get z = f(a, b).

If we set x - a = dx and y - b = dy we get z - f(a, b) = dz.

Therefore: The tangent plane touches the surface at (a, b).

Δz = change in z on the surface.

dz = change in z on the tangent plane.

Figure 11.4.5 shows Δz and dz.

11_partial_differentiation-229.gif

Figure 11.4.5


Last Update: 2010-11-25