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

First term = 12

Step-by-step 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

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