Answer : For a diagonal to be formed, 2 vertices are required. Thus in a polygon, there are 10 sides. And no. of lines can be formed are nC2, but in nC2 the sides are also included. N of them is sides.
Thus the no. of diagonals are nC2 – n
Hexagon N = 6
so no of diagonal is 6C2-6
= 9
- decagon N = 10
So no of diagonal is 10C2-10
= 35
- N= 18
So no of diagonal is 18C2-18
= 135