<s>[INST] Find volume of solid bounded by given surfaces. $z=a+x,z=-a-x,x^2+y^2=a^2$ Find volume of solid bounded by given surfaces.
$$z=a+x, \qquad z=-a-x, \qquad x^2+y^2=a^2$$

This is the solid. We can find volume of solid that has positive $z$ value and multiply by $2$. And for finding volume of that solid I thought doing double integral where domain is circle and function is that ellipse. $\int_{-a}^a dy \int_{-a}^a f(x,y)dx$. Where $f(x,y)$ is intersection of plane $z=a+x$ and $x^2+y^2=a^2$. Is my thinking right? When I want to find intersection I  substitute $x=z-a$ into   $x^2+y^2=a^2$ I get $z=\frac{a+\sqrt{a^2-y^2}}{2}$ which when I graph does not look like ellipse. Can you help?
 [/INST] Intersection of planes is a line given by, $z = a + x = - a - x \implies x = - a, z = 0$
At intersection with the cylinder, $(-a)^2 + y^2 = a^2 \implies y = 0$
So, both planes and the cylinder intersect at a single point $(-a, 0, 0)$. That means the projection of the region in xy-plane is circle $x^2 + y^2 \leq a^2, z = 0$
The integral to find volume is given by,
$ \displaystyle V = \iint_{x^2 + y^2 \leq a^2} 2 |a + x| ~ dx ~ dy$
Note that if $a \lt 0, a + x $ is always negative (or zero) over the region and if $a \gt 0, a + x$ is always non-negative over the region.
Now as $x$ is an odd function, $[f(-x) = - f(x)]$, and the region is symmetric to yz plane, its integral over $x \gt 0$ will cancel out the integral over $x \lt 0$.
So, $ \displaystyle V = 2 |a| \iint_{x^2 + y^2 \leq a^2}  dx ~ dy = 2 \pi |a|^3$
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