Formula for number of diagonals in an \(n\-sided polygon: \(\dfrac{n(n-3)}{2}\

For decagon (\(n = 10\): \(\dfrac{10 \times 7}{2} = \dfrac{70}{2} = \mathbf{35}\

This formula counts all connections between non-adjacent vertices. Total connections = \(\binom{10}{2} = 45\, minus 10 sides = 35 diagonals.