12
A decagon has 10 sides
no
a decogon
If decogon were a real word then the prefix would, could be de-, dec- or deco-. But it is not a real word and so there is no prefix.
None because there is no such figure. A decagon, on the other hand, has just one face.
it is a ten sided and ten angled figure
A decagon is a polygon with 10 sides and 10 angles. Deca is from the Greek meaning of "ten".
Decogon * * * * * A decagon, actually.
it is a ten sided and ten angled figure
there are 27 triangles in a triangle
A decagon need not have any. It can have five pairs, or two sets of 5 each, or various combinations in between.