triangle: 3sides
quadrilateral: 4 sides
pentagon: 5 sides
hexagon: 6 sides
heptagon: 7 sides
octagon: 8 sides
nonagon: 9 sides
decagon: 10 sides
hendecagon: 11 sides
dodecagon: 12 sides
triskaidecagon: 13 sides
tetradecagon: 14 sides
pentadecagon: 15 sides
hexadecagon: 16 sides
heptadecagon: 17 sides
octadecagon: 18 sides
nonadecagon: 19 sides
icosagon: 20 sides
http://thumbs.dreamstime.com/z/formulas-equations-areas-various-d-shapes-vector-29705664.jpg