Question

Number of diagonals in a decagon is

Answer 1

$$\begin{gathered} \mathrm{Number}\mathrm{of}\mathrm{sides}\mathrm{of}\mathrm{decagon}=10 \\ \mathrm{Number}\mathrm{of}\mathrm{diagonals}\mathrm{in}\mathrm{a}\mathrm{decagon}=\frac{\mathrm{n}(\mathrm{n}-3)}{2}=\frac{10(10-3)}{2}=5\times 7=35 \end{gathered}$$