Question
Find the sum of odd integers from 1 to 2001.
Solution
All the odd integers from 1 to 2001 are 1, 3, 5 , 7 , 9,.............. 2001.
Let 
denote their sum, i.e. 
Here, a = 1, d = 2, 
i.e., a + (n - 1) d = 2001 or 1 + (n - 1)2 = 2001
or 1 + 2n - 2 = 2001 or 2n = 2001 + 1 = 2002
or 
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