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2014-05-04T20:17:24+03:00
1+2+3+...+n=\frac{n*(n+1)}{2} =210
n(n+1)=420         
 n^{2} +n-420=0
Δ=1+4*420=1+1680=1681
n=\frac{-1+ \sqrt{1681} }{2}  =  \frac{-1+41}{2} =20Ω
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