Question

[n²/2+11n/2]-[(n-1)²/2+11(n-1)/2]

Answer 1

[n^2/2+11n/2]--[(n--1)^2/2+11(n--1)/2]

=[n(n+11)/2]--[(n--1)(n--11+1)/2]

=[n(n+1)--(n--1)(n--10)]/2

=(n^2+n--n^2+11n--10)/2

=(12n--10)/2

=6n--5.