# Recitation 001

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Above is not an valid proof because a proof cannot start with the proposition, no matter what.

Techniques:

• You can also prove by cases, stating that when x=0 and when x>0. (problem 2)

• You can cite other proves by using "by" word (problem 3)

• You can prove by double prove (greater than, smaller than) (problem 4)

Warnings:

• :warning: You have to say the left is what, the right is what, and the left = right. (problem.1)

• :warning: You cannot Let the thing we are trying to prove.

• :warning: some operations are irreversible. (like sqrt(x^2)=|x|) and you cannot remove absolute value from both side. (problem )

Sometimes derivatives lose information :question: why?

:question: you cannot remove absolute value from both side? TODO: do all problems

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