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Active Calculus - Multivariable

Activity 12.4.3.
Suppose we want to find the area of the bounded region \(D\) between the curves
\begin{equation*} y = 1-x^2 \quad \quad \text{ and } \quad \quad y=x-1 \end{equation*}
A picture of this region is shown in Figure 12.4.2.
Figure 12.4.2. The graphs of \(y = 1-x^2\) and \(y=x-1\)
(a)
The volume of a solid with constant height is given by the area of the base times the height. Hence, we may interpret the area of the region \(D\) as the volume of a solid with base \(D\) and of uniform height 1. That is, the area of \(D\) is given by \(\iint_D 1 \, dA\text{.}\) Write an iterated integral whose value is \(\iint_D 1 \, dA\text{.}\)
Hint.
Which order of integration might be more efficient? Why?
(b)
Evaluate the iterated integral from (a). What does the result tell you?