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when toggle format what by license comment
Sep 11, 2016 at 15:38 comment added Oai Thanh Đào Because your reference both sides are the same $(x)$, but this question left side is $(x)$ and right side is $(a)$
Sep 10, 2016 at 5:23 comment added Oai Thanh Đào I am not sure, but I think it is not proof. It is a reference.
Sep 9, 2016 at 4:07 history answered T. Amdeberhan CC BY-SA 3.0