The sum of first 20 odd natural numbers is ______.

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The sum of first 20 odd natural numbers is 400. Explanation: Let a be the first term and d be the common difference. We know that, sum of first n terms = Sn = n/2[2a + (n – 1)d] The given series is 1 + 3 + 5 + ...... First term = a = 1 Common difference = d = 3 – 1 = 2 $\therefore$ S20 = $\frac{20}{2}$ 2 $\times$ 1 + (20 – 1)2] = 10(2 + 19 $\times$ 2) = 10(40) = 400

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