Answer :

Reversing the AP, –84…, 4, 8, 12

a = –84

d = 12 – 8 = 4

a_{11} = a + (11 – 1)d

a_{11} = –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

x^{2} + 4x + 3 = 10608

x^{2} + 4x – 10605 = 0

x^{2} + 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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