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I was looking for an answer for this question, I found it here on math stackexchage, but there's something in the answer I did not understand. I tried to add a comment too the answer, but I couldn't, because I dont have reputation enough. I cannot see where that tanget came from:

$ \int \int_{\mathbb{R}}f(x,y)dxdy = 2\pi \int_{0}^{\infty}tg(t)dt = \frac{2\pi}{B^2} $

link of the answer below: https://math.stackexchange.com/a/385427/755029

Could you help me, please?

Stenio W. R. da Silva
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1 Answers1

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There's no tangent there. The abbreviation for the tangent is $\tan$, not $\operatorname{tg}$; also, when a user has a reputation of several thousand, it’s rather likely that they can properly typeset math and wouldn’t italicize a function name.

The integrand is $t\cdot g(t)$. The factor $t$ is the Jacobian for the transformation from Cartesian to polar coordinates.

joriki
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  • aaahhh, got it! Sorry. Here in Brazil some mathematicians and Physicists use this abreviation tgg for tangent. That was my confusion. Thank you very Much. – Stenio W. R. da Silva Mar 02 '20 at 21:02