Question

how many diagonals are there in a polygon having 12 sides.

Answer 1

Ok, so that fraction doesn't look very good. I'm trying to say that the formula for diagnoals in a polygon is n(n-3) over 2. Soooooooo we can draw 54 diagonals in a 12 sided figure. cool.

Answer 2

The number of diagonals (we'll label diagonals as "d") that can be drawn in a polygon of "n" sides is given by the formula: 

d = 
n(n-3)
________
2 

Ok, so that fraction doesn't look very good. I'm trying to say that the formula for diagnoals in a polygon is n(n-3) over 2. 
Let's use your 12 sided figure as our example: 
12(12-3)/2 <---- this is a fraction of 12(12-3) over 2 

12(9)/2 is the same as 12 x 9 over 2 
108/2 is the same as 108 over 2 
108/ 2 = 54 
Soooooooo we can draw 54 diagonals in a 12 sided figure