Question Number 104752 by mohammad17 last updated on 23/Jul/20
Commented by qwertyu last updated on 23/Jul/20
$${S}_{{n}} =\frac{\mathrm{51}+\mathrm{21}}{\mathrm{2}}\:\centerdot\:\mathrm{31}\:=\:\mathrm{1116} \\ $$
Answered by Dwaipayan Shikari last updated on 23/Jul/20
$${S}=\mathrm{21}+\mathrm{22}+….\:\mathrm{31}\:{terms} \\ $$$${S}=\frac{\mathrm{31}}{\mathrm{2}}\left(\mathrm{2}.\mathrm{21}+\mathrm{30}\right)=\mathrm{31}.\mathrm{36}=\mathrm{1116} \\ $$
Commented by mohammad17 last updated on 23/Jul/20
$${thank}\:{you}\:{sir} \\ $$