Question

The 4th term of an arithmetic sequence is 24 and the 12th term is 56, what is the first term?​

Answer 1

First term = 12
Solution:
A4 = 24
A12 = 56
An = A1 + (n-1)d; this defines the formula of an arithmetic series
A4 = 24 = A1 + (4-1)d
A4 = 24 = A1 + 3d
A12 = 56 = A1 + (12-1)d
A12 = 56 = A1 + 11d
Subtract A4 from A12 to get d (distance)
(56 = A1 + 11d) - (24 = A1 +3d)
32 = 8d
d = 4
Substitute d = 4 into A4
A4 = 24 = A1 + 3(d)
24 = A1 + 3(4)
24 = A1 + 12
24 - 12 = A1
12 = A1

Answer 2

First term = 12

Explanation:

A4 = 24

A12 = 56

An = A1 + (n-1)d; this defines the formula of an arithmetic series

A4 = 24 = A1 + (4-1)d

A4 = 24 = A1 + 3d

A12 = 56 = A1 + (12-1)d

A12 = 56 = A1 + 11d

Subtract A4 from A12 to get d (distance)

(56 = A1 + 11d) - (24 = A1 +3d)

32 = 8d

d = 4

Substitute d = 4 into A4

A4 = 24 = A1 + 3(d)

24 = A1 + 3(4)

24 = A1 + 12

24 - 12 = A1

12 = A1