Question #74142

1 Answer
Alan P.
Jan 13, 2018

Depending upon interpretation of expression:
the 12th term might be #78#

Explanation:

If the intent was
#color(white)("XXX")a_n=1/2 n(n+1)color(white)("xxxxx")#...doubtful, but maybe
then
#color(white)("XXX")a_12 = 1/2 * 12 * 13 =78#

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