Answer :

a5= a + (n -1) d

2 = -14 + 4d


16 = 4d


d = 4


Sn = [2a + (n – 1) d]


40 = [2(-14) + (n -1) (4)]


40 = 12n2 – 16n


2n2 – 16n -40 = 0


n2 – 8n -20 = 0


n2 – 10n + 2n – 20 = 0


(n – 10) (n + 2) = 0


n = 10 and n = -2 (not possible)


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