Question

find the sum of the first 15 multiples of 8.​

Answer 1

Answer:

960

Step-by-step explanation:

According to the question,

8+16+24+32+....+120

It is in A.P.

Here,

a = 8, d= t2. - t1 = 16 - 8 = 8

l = 120, n= 15

Sn= n/2[2a+(n-1)d]

= 15/2[2(8) + (15-1)8]

= 15/2 [16 + 14×8]

= 15/2 (16+112)

= 15/2 (128)

= 15×64

= 960

Therefore, the sum of first 15 multiples of 8 is 960

Answer 2

Answer:

a = 8

n = 15

d = 8

So,Sn = n/2 [2a +(n-1)d)

= 15/2 (2×8 + (14)8]

= 15/2 [16 +112]

= 15/2 [128]

= 15 [64]

= 960