`sum_(n=11)^30 n-sum_(n=1)^10 n`

The sum formula for an arithmetic sequence is

`Sn=n/2(a_1+a_n)`

`S_20=20/2(11+30)=10(41)=410`

`S_10=10/2(1+10)5(11)=55`

`sum_(n=11)^30 n-sum_(n=1)^10 n=410-55=355`

`sum_(n=11)^30 n-sum_(n=1)^10 n`

The sum formula for an arithmetic sequence is

`Sn=n/2(a_1+a_n)`

`S_20=20/2(11+30)=10(41)=410`

`S_10=10/2(1+10)5(11)=55`

`sum_(n=11)^30 n-sum_(n=1)^10 n=410-55=355`