The sum of the first 18 terms of the series 3, 6, 9, 12, 15, ....... is:

  1. 513
  2. 413
  3. 313
  4. 516

Answer (Detailed Solution Below)

Option 1 : 513
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Detailed Solution

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Concept: 

Sum of consecutive numbers from 1 to n: \(\rm1 + 2 + 3 +\ ...\ + n = \dfrac{n(n+1)}{2}\).

Calculation:

The sum of the first 18 terms of the given series can be written as:

3 + 6 + 9 + 12 + 15, ....... 

⇒ 3(1 + 2 + 3 + 4 + 5 + ....)

\(\frac{3 \times 18 \times 19}{2} = 513\) 

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