True. A series is arithmetic if its terms come from an arithmetic sequence.
And a sequence is said to be arithmetic any of its two consecutive terms differ by a fixed difference, common to any consecutive pair.
As you can see, in this case, all terms differ by 2, so the sequence
[tex] 6,\ 8,\ 10,\ 12,\ \ldots,\ 4+2n [/tex]
is arithmetic, and thus the series
[tex] 6 + 8 + 10 + 12 + \ldots + (4+2n) [/tex]
is arithmetic as well.
