Question

In an arithmetic series, the sum of the first 12 terms is equal to ten times the sum of the first 3 terms. If the first term is 5, find the common difference and the value of S20.

Answer 1

Answer:

Step-by-step explanation:

The formula for determining the sum of the first n terms of an arithmetic sequence is expressed as

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

Where

n represents the number of terms in the arithmetic sequence.

d represents the common difference of the terms in the arithmetic sequence.

a represents the first term of the arithmetic sequence.

If a = 5, the expression for the sum of the first 12 terms is

S12 = 12/2[2 × 5 + (12 - 1)d]

S12 = 6[10 + 11d]

S12 = 60 + 66d

Also, the expression for the sum of the first 3 terms is

S3 = 3/2[2 × 5 + (3 - 1)d]

S3 = 1.5[10 + 2d]

S3 = 15 + 3d

The sum of the first 12 terms is equal to ten times the sum of the first 3 terms. Therefore,

60 + 66d = 10(15 + 3d)

60 + 66d = 150 + 30d

66d + 30d = 150 - 60

36d = 90

d = 90/36

d = 2.5

For S20,

S20 = 20/2[2 × 5 + (20 - 1)2.5]

S20 = 10[10 + 47.5)

S20 = 10 × 57.5 = 575