Find the sum of the first 15 multiples of 8.

Monjur Alahi

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Salim Ahmed Kawsar

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The multiples of 8 are
8, 16, 24, 32…
These are in an A.P., having first term as 8 and common difference as 8.
Therefore, a = 8
d = 8
S15 = ?
Sn = n/2 [2a + (n - 1)d]
S15 = 15/2 [2(8) + (15 - 1)8]
= 15/2[6 + (14) (8)]
= 15/2[16 + 112]
= 15(128)/2
= 15 × 64
= 960 

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Salim Ahmed Kawsar

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The answer is 960

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Salim Ahmed Kawsar

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a=8, d=8 and n=15
`S_15=n/2(2a+(n-1)d)`
`S_15=15/2(2(8)+(15-1)(8))`
`S+15=960`

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