Correct Answer: Option B
81 = a + (n-1)d = 1 + (n-1)4
81 - 1 = (n-1)4
80 = (n-1)4
(n-1) = 80/4 = 20
n-1 = 20
n = 21
now,
S= n[2a + (n-1)d]/2
= 21[2.1 + (21-1)4]/2
= 861
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