How would you calculate the surface area of the portion of the sphere $x^2 + y^2+ z^2 = 16z$ that lies within the paraboloid $z = x^2 + y^2$.

You are watching: The part of the sphere x2 + y2 + z2 = 4z that lies inside the paraboloid z = x2 + y2.

Points common to the sphere and paraboloid satisfy the equation $z + z^2 = 16z$, so there we have either z = 0 or z = 15. The former corresponds to the origin (0, 0, 0) which lies in both surfaces where they both have z = 0 as a tangent plane.

However I am not sure how to continue. Could anyone please guide me through how you would attempt this question? thank you


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Simplify the task by re-centering the circle with $w=z-8$. Then, we have the two equations,

$$x^2+y^2+w^2=64,\>\>\>\>\>x^2+y^2=w+8$$

which intersect at

$$w^2+w-56=0\implies w = 7,\>\>\>x^2+y^2=15$$

The surface integral is,

$$I = \int_{A} \sqrt{1+(w_x")^2+(w_y")^2}dA=\int_{A} \frac 8wdA$$

Evaluate the integral in polar coordinates,

$$I = 8\int_0^{2\pi} d\theta\int_0^{\sqrt{15}} \frac{rdr}{\sqrt{64-r^2}}=-16\pi\sqrt{64-r^2}|_0^\sqrt{15}=16\pi$$


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Both the sphere and the paraboloid are rotationally symmetric around the $z$-axis, so their intersection must be one or more circles (again symmetric around the $z$-axis). You can now determine a parametrization of the part of the sphere between those circles and integrate the surface element.

For example you can say $s(u,v) = (c_x + r\cos u\cos v, c_y + r\sin u\cos v, c_z + r\sin v)$ for proper values of radius $r$ and center $c = (c_x,c_y,c_z)$, then choose the domains of $u$ and $v$ as necessary to cover exactly the parts of the sphere you need.


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This figure should help:

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Calculate the angle $\phi$ with respect to the $z$ axis of the (red) boundary, then you can easily calculate the area of the (outer) sphere.

Knowing its $z$ height allows you to calculate the area of the paraboloid too.


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