Discussion
Find the sum of the first 20 terms of the A.P. 1, 4, 7, 10, ...
Answer: A
Here, \(a = 1\), \(d = 3\), and \(n = 20\). The formula for the sum of the first n terms of an A.P. is \(S_n = \frac{n}{2}[2a + (n-1)d]\). So, \(S_{20} = \frac{20}{2}[2(1) + (20-1)3] = 10[2 + 19 \times 3] = 10[2+57] = 10 \times 59 = 590\).
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