Question 129203
The number of {{{diagonals=d}}} in a polygon is: {{{d = (1/2) N(N-3)}}}

you have a pentagon, then {{{N=5}}}

so, {{{d = (1/2) 5(5-3)}}}
{{{d = (1/2) 5(2)}}}
{{{d = (1/2) 10 }}}
{{{d = 5 }}}

The number of {{{triangles=T}}} (when you draw all the diagonals from {{{one}}}{{{ vertex}}}) in a polygon is: {{{T = (N - 2)}}}

the number of {{{triangles=T}}} 

{{{T = (5 - 2)}}}

{{{T = 3}}}