it depends on what you mean by a diagonal. If you take a diagonal to mean a line from one corner to the "opposite" corner, then a nonagon doesn't have any, and a decagon has five. The reason a nonagon doesn't have any, is that the corners aren't opposite each other.
There are 35 diagonals in a 10 sided decagon
A tetra-decagon is a 14 sided polygon having 77 diagonals
35
A nonagon has 9 sides whereas a decagon has 10 sides
27
A Nonagon has 36 diagonals
A nonagon has 9 sides and a decagon has 10 sides
24 diagonals
It has 27 diagonals
There are 35 diagonals in a 10 sided decagon
9 diagonals
27 diagonals
A tetra-decagon is a 14 sided polygon having 77 diagonals
27
35.
35
35