Question

Find the sum of the first 15 multiples of 8.

NCERT Class X
Mathematics - Mathematics

Chapter _ARITHMETIC PROGRESSIONS

Answer 1

hi there,

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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 = 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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hope the helped u,
cheers

Answer 2

Hello,

Question;-

Find the sum of the first 15 multiplies of 8

Method of Solution;-


Let to be a is first term and 'd'" is common Difference and l is last term of Given Arithmetic Sequence or Progression;-

Arithmetic Sequence or Progression which are given below;-

Arithmetic Sequence or Progression;-

8,16,24,32....

Here,

First term= 8

CommOn Difference=8

Number of terms=15

We know that Formula of Summation of Arithmetic Sequence or Progression.

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

S15=15/2(2x8+(15-1)8

  =15/2(16+(14)8)

  =15/2(16+112)

  =15/2 x 128

  =15 x 64

 =960


Hence ,960 are sum of the first 15 multiples of 8.