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Question
The Sum of first five multiples of 3 is ______.
Options
45
55
15
75
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Solution
The Sum of first five multiples of 3 is 45.
Explanation:-
The given sequence is 3, 6, 9,...
Here,
a = 3
d = 3
n = 5
Therefore,
`S_n = n/2 [2a + (n - 1) d]`
`S_5 = 5/2 [2a + (5 - 1) d]`
= `5/2 (2(3) + 4(3))`
= `5/2 (6 + 12)`
= `5/2 (18)`
= 5 × 9
= 45
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