alleryStart learning →The sum of the first 20 terms of the AP $1,\ 3,\ 5,\ 7,\ \ldots$ is
A210
B420
C400
D380
Answer & Solution
Correct answer: C. 400
1. a = 1, d = 2, n = 20.
2. $S_{20} = \tfrac{20}{2}[2(1) + 19(2)] = 10[2 + 38] = 10(40)$.
3. $S_{20} = 400$ (the sum of the first 20 odd numbers is $20^2 = 400$).
_Source: Karnataka SSLC (KSEEB) Class 10 Mathematics, Ch1 'Arithmetic Progressions' (sum of n terms)_
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