Answer :

Reversing the AP, –84…, 4, 8, 12


a = –84


d = 12 – 8 = 4


a11 = a + (11 – 1)d


a11 = –84 + 10 × 4 = –44


OR


a = 1


d = 5 – 1 = 4


Last term = a + (n – 1)d


x = 1 + (n – 1)4


x – 1 = 4n – 4


4n = x + 3


n =


Some of n terms = 1326


(x + 1) = 1326


x2 + 4x + 3 = 10608


x2 + 4x – 10605 = 0


x2 + 105x – 101x – 10605 = 0


x(x + 105) – 101 (x + 105) = 0


(x –101) (x + 105) = 0


x = 101, x = –105


Neglecting x = –105 as AP is increasing


x = 101


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