Question 1131843
A decagon is a ten sided polygon.

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from right triangle {{{APO}}} we have that apothem {{{a}}} is {{{sqrt(r^2-(s/2)^2)}}}
and  {{{sin(144/2=72)}}}º

{{{a = r sin(72)}}}
{{{a = 0.951 r }}}-> since {{{r=10}}}
{{{a = 9.51056516295153572}}}
 
then, side {{{s}}} will be
{{{a=sqrt(r^2-(s/2)^2)}}}
{{{9.51056516295153572=sqrt(10^2-(s/2)^2)}}}
{{{9.51056516295153572^2=100-(s/2)^2}}}
{{{(s/2)^2=100-9.51056516295153572^2}}}
{{{(s/2)^2=9.5491502812526288170334064935640816}}}
{{{(s/2)=sqrt(9.5491502812526288170334064935640816)}}}
{{{(s/2)=3.0901699437494742446065683660420445}}}
{{{s=3.0901699437494742446065683660420445*2}}}
{{{s=6.180339887498948489213136732084089}}}
{{{s=6.18m}}}