# Find the sum of odd integers from 1 to 2001.

Sn = 1 + 3 + 5 + 7 + ….. + 2001

Now a = 1

d = 3 – 1 = 2, l = 2001

Let tn = 2001

Therefore a + (n + 1) d = 2001

or 1 + (n – 1)(2) = 2001

or (2n – 1) = 2000

or n – 1 = 1000

or n = 1001

We have Sn =

or S1001 =
= = 1001 × 1001
= 1002001

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