Consult any textbook on Euclidean geometry.
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theorem always needs proof
There is no theorem with the standard name "1.20". This is probably a non-standard name from a textbook which is either the 20th theorem in the first chapter or a theorem of the 20th section of the first chapter.
The midpoint theorem says the following: In any triangle the segment joining the midpoints of the 2 sides of the triangle will be parallel to the third side and equal to half of it
He didn't write it. What he did was to write in the margin of a book that he had a proof but there was not enough space to write it there.
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").