Answer :

Sum of first 20 odd natural numbers is 1 + 3 + 5 + 7 + … + 39.


This forms an arithmetic series with first term = a = 1


and common difference = d = 2


Sum of n terms of this arithmetic series is given by:


Sn = [2a + (n - 1)d]


Therefore sum of 20 terms of this arithmetic series is given by:


S20 = [2(1) + (20 - 1)(2)]


= 10 [2 + 38]


= 10 × 40


= 400

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