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Question
The sum of first 20 odd natural numbers is
Options
100
210
400
420
Solution
Let a be the first term and d be the common difference.
We know that, sum of first n terms = Sn = \[\frac{n}{2}\][2a + (n − 1)d]
The given series is 1 + 3 + 5 + ......
First term = a = 1.
Common difference = d = 3 − 1 = 2
∴ S20 = \[\frac{20}{2}\] 2 × 1 + (20 − 1)2]
= 10(2 + 19 × 2)
= 10(40)
= 400
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