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प्रश्न
Find the sum of first 20 terms of the following A.P. : 1, 4, 7, 10, ........
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उत्तर
Let a be the first term and d be the common difference of the given A.P. Then, we have a = 1 and d = 3.
We have to find the sum of 20 terms of the given A.P.
Putting a = 1, d = 3, n = 20 in
`S_n = \frac{n}{ 2 } [2a + (n - 1) d]`
∴ `S_20 = \frac {20}{2} [2 × 1 + (20 - 1) × 3]`
= 10 (2 + 57)
= 10 × 59
= 590
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