Question

Why do \(z=f(x, y)\) and \(z=g(x, y)=f(x, y)+h,\) for some real number \(h\), have the same surface area over a region \(R ?\)

Short answer

The surface areas are the same because adding a constant \( h \) does not affect partial derivatives.

Step-by-step solution

Understanding the Problem

We have two functions: \( z = f(x, y) \) and \( z = g(x, y) = f(x, y) + h \), where \( h \) is a constant. Both need to be evaluated over a region \( R \). We are asked to compare their surface areas.

Recall Surface Area Formula

The formula for the surface area of \( z = f(x, y) \) over a region \( R \) is given by: \[ A = \iint_R \sqrt{1 + \left( \frac{\partial f}{\partial x} \right)^2 + \left( \frac{\partial f}{\partial y} \right)^2} \, dx \, dy \]

Apply the Formula to Each Function

For \( z=g(x, y) = f(x, y) + h \), calculate the partial derivatives with respect to \( x \) and \( y \):\( \frac{\partial g}{\partial x} = \frac{\partial f}{\partial x} \) and \( \frac{\partial g}{\partial y} = \frac{\partial f}{\partial y} \). The surface area formula for \( g(x, y) \) becomes: \[ \iint_R \sqrt{1 + \left( \frac{\partial g}{\partial x} \right)^2 + \left( \frac{\partial g}{\partial y} \right)^2} \, dx \, dy \]

Comparing the Two Surface Area Formulas

The expressions \( \sqrt{1 + \left( \frac{\partial f}{\partial x} \right)^2 + \left( \frac{\partial f}{\partial y} \right)^2} \) and \( \sqrt{1 + \left( \frac{\partial g}{\partial x} \right)^2 + \left( \frac{\partial g}{\partial y} \right)^2} \) are identical since partial derivatives do not change when adding a constant.Therefore, the surface areas of \( f \) and \( g \) over \( R \) are the same.

Key concepts

Partial Derivatives

When dealing with functions of multiple variables, partial derivatives help us understand how a function changes with respect to one variable while keeping the others constant. For a function \(z = f(x, y)\), the partial derivative with respect to \(x\), denoted \(\frac{\partial f}{\partial x}\), represents the rate of change of \(f\) as \(x\) changes and \(y\) remains fixed. Similarly, the partial derivative with respect to \(y\), \(\frac{\partial f}{\partial y}\), shows how \(f\) changes as \(y\) increases while \(x\) is constant.

• Partial derivatives are crucial when calculating the surface area of a function, as they provide the slopes needed in the surface area formula.
• In the context of surface area, they help determine the orientation and steepness of the surface, affecting the area computation.

With the function \(z = g(x, y) = f(x, y) + h\), adding a constant \(h\) does not change its partial derivatives compared to \(f(x, y)\). This is because differentiating a constant yields zero, and the derivatives continue to depend only on \(f(x, y)\). Understanding partial derivatives thus simplifies our task when comparing surface areas of functions differing by a constant, as seen in both \(f(x, y)\) and \(g(x, y)\).

Double Integrals

Double integrals extend the concept of integration to functions of multiple variables, allowing us to compute accumulated quantities over a region. In the case of surface area, a double integral helps sum up all the infinitesimal pieces of surface area over the region \(R\). If \(A\) represents surface area, it is computed as:\[ A = \iint_R \sqrt{1 + \left( \frac{\partial f}{\partial x} \right)^2 + \left( \frac{\partial f}{\partial y} \right)^2} \, dx \, dy \]

• The double integral essentially covers region \(R\), aggregating all tiny surface areas influenced by the partial derivatives of the function.
• It allows for a full accumulation over not just linear paths, but entire regions defined by two variables.

Utilizing double integrals is essential in multivariable calculus, particularly for calculating surface areas or volumes where changes happen across a region rather than along a line. In our case, when comparing functions \(f(x, y)\) and \(g(x, y)\), after establishing that partial derivatives remain unaffected by a constant, the double integrals ensure their surface areas stay identical over the same region.

Constant Function Addition

Adding a constant to a function, such as changing \(z = f(x, y)\) to \(z = g(x, y) = f(x, y) + h\), is straightforward in theory but requires understanding its implications in calculus.

• This constant \(h\) represents a vertical shift in the graph of the function, altering its position without changing its shape or the nature of its surface.
• The key takeaway is that since this shift does not affect the function's partial derivatives, the geometric properties essential for surface area calculation remain unchanged.

This is why the surface area, which depends on the slopes given by those derivatives, stays the same despite adding a constant. The constant addition demonstrates a fundamental principle: transformations that do not influence directional rates of change will not alter cumulative quantities like surface area. Therefore, for functions \(f\) and \(g\), their surface areas over any given region \(R\) are identical, modeling the principle from calculus that vertical translations do not impact the computation of areas.