`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`
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