Question

find the sum of first fifteen multiples of 8

Answer 1

The first 8 multiples of 8 are
8, 16, 24, 32., 40, 48, 56,64

These are in an A.P., having first term
as 8 and common difference as 8.
Therefore, a=8
d=8
S15=?
=Sn h- 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 x 64
=960


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