Ok, so here are the informations.
Area: $770 \;\mathrm{cm}^2$
bottom to top: $30.8$
height: half of $30.8$ = $15.4$
Find the perimeter?
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What kind of decagon? Regular? – André Nicolas Feb 04 '15 at 19:57
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Sorry, yes regular. How do you exactly know the side length is my question also. – naaame Feb 04 '15 at 19:58
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The perimeter is $10s$ where $s$ is the side length. Connect the centre to two adjacent vertices. You can (trig) express the area of the triangle in terms of $s$. – André Nicolas Feb 04 '15 at 20:04
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Yeah, I don't quite understand. – naaame Feb 04 '15 at 20:08
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The question has already been answered elsewhere. Odd question, since if we know the height we can compute the area, and vice-versa. – André Nicolas Feb 04 '15 at 20:21
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Take a look at this page: http://www.mathwords.com/a/area_regular_polygon.htm – Fermat Feb 04 '15 at 20:26