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I’m doing a lot of double integrals to find surface area problems, and I don’t think I’m setting them up quite right. For example,
“Find the surface area of the portion of the sphere x^2 + y^2 + z^2 = 25 inside the cylinder x^2 + y^2 = 9.”
I converted the sphere to a function of z: \sqrt{25 - x^2 - y^2}
Then I found the partial derivatives:
fx = \frac{-x} {\sqrt{25 - x^2 - y^2}}
fy = \frac{-y} {\sqrt{25 - x^2 - y^2}}
Then I set up the integral:
SA = \int \int \sqrt{1 + (fx)^2 + (fy)^2} dx dy
SA = \int \int \sqrt{1 + \frac{x^2} {25 - x^2 - y^2} + \frac{y^2} {25 - x^2 - y^2}} dx dy
Adding fractions, I get:
SA = \int \int \sqrt{ \frac{25} {25 - x^2 - y^2} } dx dy
Converting to polar, I get:
SA = \int_0^3 \int_0^{2\pi} r\sqrt{ \frac{25} {25 - r^2} } d\theta dr
(That pi should be a part of the integral range, but I can’t find the fraking index of forum code.)
I can't think of a way to integrate this, so I think I didn't set it up right.
“Find the surface area of the portion of the sphere x^2 + y^2 + z^2 = 25 inside the cylinder x^2 + y^2 = 9.”
I converted the sphere to a function of z: \sqrt{25 - x^2 - y^2}
Then I found the partial derivatives:
fx = \frac{-x} {\sqrt{25 - x^2 - y^2}}
fy = \frac{-y} {\sqrt{25 - x^2 - y^2}}
Then I set up the integral:
SA = \int \int \sqrt{1 + (fx)^2 + (fy)^2} dx dy
SA = \int \int \sqrt{1 + \frac{x^2} {25 - x^2 - y^2} + \frac{y^2} {25 - x^2 - y^2}} dx dy
Adding fractions, I get:
SA = \int \int \sqrt{ \frac{25} {25 - x^2 - y^2} } dx dy
Converting to polar, I get:
SA = \int_0^3 \int_0^{2\pi} r\sqrt{ \frac{25} {25 - r^2} } d\theta dr
(That pi should be a part of the integral range, but I can’t find the fraking index of forum code.)
I can't think of a way to integrate this, so I think I didn't set it up right.
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