Question

74)
If the total number of diagonals of an n-gon is 77, then n = ________ ?

a) 7
b) 8
c) 12
d) 14

Answer 1

help me please. thanks

Answer 2

Total no. of diagonals = nC2 - n.
=>
nC2 - n = 77
(n(n-1) - 2n) = 154
n = 14

Answer 3

It is nC2 - n because total diagonals can be understood as :
Select any two points from all of them.
Subtract n as A polygon has n sides and you dont want those in diagonals.