n := 4

n(n+1)/2

Sum[ i, {i,1,4}]

Sum[i^2-17i, {i,4, 10}]

Sums of the first few odd numbers:

Sum[ 2k-1, {k,1,1}]

Sum[ 2k-1, {k,1,2}]

Sum[ 2k-1, {k,1,3}]

Sum[ 2k-1, {k,1,4}]