Question

if the n th term of an A.P is (2n+1) ,then the sum of its first three term will be

Answer 1

$$\begin{gathered} \mathrm{Given},\mathrm{nth}\mathrm{term}\mathrm{of}\mathrm{an}\mathrm{A}.\mathrm{P}.\left({\mathrm{T}}_{\mathrm{n}}\right)=2\mathrm{n}+1 \\ \mathrm{So}, \\ {\mathrm{T}}_1=2\times 1+1=3 \\ {\mathrm{T}}_2=2\times 2+1=5 \\ {\mathrm{T}}_3=2\times 3+1=7 \\ \mathrm{Now}, \\ {\mathrm{S}}_3={\mathrm{T}}_1+{\mathrm{T}}_2+{\mathrm{T}}_3=3+5+7=15 \\ \mathrm{Thus}\mathbf{,}\mathbf{}{\mathbf{S}}_{\mathbf{3}}\mathbf{=}\mathbf{15} \end{gathered}$$