I don't know how to start the question. The title is self explanatory. How to approach and make a formal proof?
How to prove that r is a double root if and only if it is a root of a polynomial and of its derivative.
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derivatives
roots
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0@Kannappan I vote not to close as dup of *said* question - which is not an *exact* dup (but is a *related* question). Please be more careful about dup closing, since it seems often others vote to close without even looking at the proposed dup. There may be another exact dup though. – 2012-04-04
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0@BillDubuque I am extremely sorry. – 2012-04-04