[tex]\\ \rm\longrightarrow 1+2\dots +20[/tex]
Use Summation formula
[tex]\\ \rm\longrightarrow S_n=\dfrac{n(n+1)}{2}[/tex]
[tex]\\ \rm\longrightarrow S_{20}=\dfrac{20(20+1)}{2}[/tex]
[tex]\\ \rm\longrightarrow S_{20}=\dfrac{20(21)}{2}[/tex]
[tex]\\ \rm\longrightarrow S_{20}=\dfrac{420}{2}[/tex]
[tex]\\ \rm\longrightarrow S_{20}=210[/tex]
