Question

The 3-rd term of the arithmetic progression is equal to 1. The 10-th term of it is three times as much as

the 6-th term. Find the first term and the common difference. (Hint: Use the formula for the n-th term
of the progression and write what is given in the problem using this formula.)

Answer 1

First term: 3

Common difference: -1

Explanation:

3rd term: 1

6th term: x

10th term: 3x

Formula for the nth term using the common difference d and the first term a1:

an = a1 + (n - 1)d

Third term: a3 = a1 + (3 - 1)d = a1 + 2d = 1

6th term: a6 = a1 + (6 - 1)d = a1 + 5d = x

10th term: a10 = a1 + (10 - 1)d = a1 + 9d = 3x

a1 + 2d = 1 Eq. 1

a1 + 5d = x Eq. 2

a1 + 9d = 3x Eq. 3

Eq. 2 - Eq. 1

3d = x - 1

Eq. 3 - Eq. 2

4d = 2x

x = 2d

3d = x - 1

Substitute 2d for x.

3d = 2d - 1

d = -1

a1 + 2d = 1

a1 + 2(-1) = 1

a1 = 3

Check:

Here are the first 10 terms:

3, 2, 1, 0, -1, -2, -3, -4, -5, -6

The third term is 1. Correct

The tenth term (-6) is 3 times the 6th term (-2). Correct

Answer:

First term: 3

Common difference: -1