Definition or Theorem ?
Let us be careful not to confuse definition and theorem.
Examples
- The following is not a definition, but a theorem:
"The right angle is the angle of magnitude 90 degree."
The expression of the definition is not so easy to make. But, anyway, its key tone is like this :
"The angle that is a quater of the round is called the right angle."
And this implies that "the right angle is of magnitude 90 degree", because the round is of magnitude 360 degree.
Here "the round is of magnitude 360 degree" is the definition of "degree". Why "360"? The answer is not in the mathematics, or logic. It is in a history, that is, the ancient civilization of Mesopotamia.
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