Question

first 2 terms of an ap -1 and 1, then 21st term is??​

Answer 1

a1 = a

a2 - a1 = 1 - (-1) = d

d = 2 [ d = common difference]

an = a + (n - 1)d

a21 = -1 + (21 - 1)2

a21 = -1 + 40

a21 = 39