Some solvings on proof of some basic theory

Is is possible to proof that |x-y| ≤ |x|+|y|?

Hello, how can I proof the next theorem? I have a linear transformation T(X) that can be expres...

Make all solvings neatly clear

looking at this proof of the pythagorean theorem, which statement(s) would be true? check all that a...

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