Question 1000017
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Your equation is 

{{{(n*(n-3))/2}}} = {{{5n}}}.

Note that {{{(n*(n-3))/2}}} is the number of diagonal of a convex n-gon.

Simplify this equation:

{{{n^2 - 3n}}} = {{{10n}}},

{{{n^2}}} = {{{13n}}}.

The solution is n = 13.
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