Question

The fifth term of an arithmetic sequence is 23 and the 12th term is 72? Determine the first three terms of the sequence and the nth term​

Answer 1

the first term is a1

the distance between the numbers is d

we have:

a5 = a1 + 4d = 23

a12 = a1 + 11d = 72

so we have the equation:

a1 + 4d = 23

a1 + 11d = 72

=> a1 = -5

d = 7

=> an = -5 + 7(n - 1) = 7n - 12

with an = 7n - 12

=> a2 = 2

a3 = 9

so the first three terms is -5; 2; 9