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Q50
(CISF/2022)
Miscellaneous & General Knowledge › Important Days, Places & Events
The sum of the numbers 3, 6, 9, 12, ... up to the 20th term is :
Result
Your answer:
—
·
Correct:
C
Explanation
The given sequence 3, 6, 9, 12, ... is an Arithmetic Progression (AP) because the difference between consecutive terms is constant.
- First term (a) = 3
- Common difference (d) = 6 - 3 = 3
- Number of terms (n) = 20
The formula for the sum of the first n terms (Sn) of an AP is:
Sn = n/2 [2a + (n - 1)d]
Substituting the values:
S20 = 20/2 [2 × 3 + (20 - 1) × 3]
S20 = 10 [6 + 19 × 3]
S20 = 10 [6 + 57]
S20 = 10 [63]
S20 = 630
Thus, the sum of the first 20 terms is 630. Therefore, option C is correct.
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