[n²/2+11n²/2]-[(n-1)²/2+11(n-1)/2]
=n^2/2 + 11n^2/2 - ((n-1)^2/2 + 11(n-1)/2)
=n^2/2 + 11n^2/2 - ((n-1)^2 + 11(n-1)/2)
=n^2/2 + 11n^2/2 - ((n-1)(n-1)^(2-1) + 11/2)
=n^2/2 + 11n^2/2 - ((n-1)(n-1+11)/2)
=n^2/2 + 11n^2/2 - ((n-1)(n+10)/2)
=( n^2 + 11n^2 - (n-1)(n+10) )/2
=( 12n^2 - (n^2-9n+10) )/2
=( 11n^2 -9n+10 )/2
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