I will give a link that explains and proves the theorem.
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Theorem 8.11 in what book?
in this theorem we will neglect the given resistance and in next step mean as second step we will solve
You can find an introduction to Stokes' Theorem in the corresponding Wikipedia article - as well as a short explanation that makes it seem reasonable. Perhaps you can find a proof under the links at the bottom of the Wikipedia article ("Further reading").
Well, this will depend on the length of the sides of the triangle for what postulate or theorem you will be using.
SAS