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- Homework Statement
- Please see image

- Homework Equations
- Please see image

I don't understand why we could not use the 1/4 of the circle lying on the xy plane as R. In the exercise it is not explained.

The idea would be taking the arc length. I know it is not easier than making your projection on the xz plane, but just wondering if this is possible. I guess it is not, but why...

**EDIT**

I've just realized there's a big issue if we select our infinitesimal patch of area to lie on the xy plane in this problem. Let's show it:

The unit normal vector is:

Then we'd get:

Note that nk yields zero, so this method is discarded.

Thanks.

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