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").
Parts of formal proof of theorem?
No. A corollary goes a little bit further than a theorem and, while most of the proof is based on the theorem, the extra bit needs additional proof.
Theory_of_BPT_theorem
When a postulate has been proven it becomes a theorem.
a theorem that follows directly from another theorem or postulate, with little of no proof
Theorems is what is proven with the geometric proof.
theorem always needs proof
o.o
theorem
A visual proof of the Pythagorean theorem, claimed to have been devised by the great genius of the Renaissance, Leonardo da Vinci.
Theorem 3.9. If two lines are perpendicular, then they intersect to form 4 right angles. You would do a proof by using your hands.
There is no single statement that describes a geometric proof.