How do we prove that $a^2+b^2=c^2$ for a right angled triangle? [closed]

How do we prove that $a^2+b^2=c^2$ for a right angled triangle whose hypotenuse measures $c$ units and the other sides measure $a$ units and $b$ units?

NOTE: $3^2+4^2=5^2$

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There are many proofs easily found by googling "Proof of Pythagoras' theorem". I suggest trying that. –  Ragib Zaman Aug 18 '12 at 14:21
As remarked by Ragib, there are many proofs. It depends on what knowledge you have available and want to use. The ones which I like best are nr. 3 and nr. 4 on the following page: cut-the-knot.org/pythagoras/index.shtml –  user20266 Aug 18 '12 at 14:23
Here is another Math.SE thread containing many proofs. –  Ragib Zaman Aug 18 '12 at 14:36
Then it is a duplicate? I'm fine with closing it cause of this. –  user20266 Aug 18 '12 at 14:37