Explanation: Since the given AP is a decreasing progression where an – 1>an,it is bound
to have negative values in the series. Sn is maximum for n = 9 or n = 10 since T10 is
0(S10 = S9 = Smax = 90). The sum of 78 can be attained by either adding 6 terms or 13
terms so that negative terms from T11 onward decrease the maximum sum to 78.
How many terms of the AP 18 16, 14, 12, …. are needed to give the sum 78? Explain the double answer.
How many terms of the AP 18 16, 14, 12, …. are needed to give the sum 78? Explain the double answer.