how many diagonals from a vertex a heptagon have
5 triangles are formed.
If you mean "How many diagonals can be drawn from one vertex of a figure with 16 sides", the formula is n-3, where "n" being the number of sides of the figure. So 16-3 = 13 diagonals that can be drawn from one vertex.
13 The correct answer is 12. From any one vertex, you can draw a diagonal to all but 3 vertices: the vertex itself and the next vertex on either side of your vertex (these would be sides of your shape, not diagonals).
If all of the diagonals are drawn from a vertex of an octagon, how many triangles are formed
Depends on the shape.
It is 18 diagonals
A diagonal of a polygon is a segment drawn from one vertex to another non-adjacent vertex in a polygon. This leaves 32 diagonals that can be drawn from one vertex in a 35 sided polygon.
n-3 from each vertex.
A regular 12 sided dodecagon has 54 diagonals.