To Find: Number of terms required to make the sum 78.
Here a = 18, d = - 2
Let n be the number of terms required to make the sum 78.
⇒ 78 × 2 = 36n - 2n2 + 2n
⇒ n2 - 19n + 78 = 0
⇒ n2 - 6n - 13n + 78 = 0
⇒ n(n - 6) - 13(n - 6) = 0
⇒ (n - 13)(n - 6) = 0
Either n = 13 or n = 6
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.