The sum of first 20 odd natural numbers is

Foundation

Mathematics Foundation

Sum of n terms of an AP

Question

100

210

400

420

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S₂₀ = 1 + 3 + 5 + 7 + ...... up to 20 terms.

Here a = 1, d = 2. So, T₂₀ = (a + 19d) = (1 + 19 × 2) = 38 = $l$ .

$∴ S_{10} = \frac{n}{2} (a + ℓ) = \frac{20}{2} (1 + 39) = 400$