Question

The third term in an arithmetic sequence is 9 and the fifth term is 21 . If the first term if a1, which is an equation for the nth term of this sequence?

A:an=6n-9
B:an=14n+6
C:an=6n+14
D:an=9n-6

Answer 1

A: an = 6n - 9

Explanation:

third term in an arithmetic sequence is 9

a3 = a + 2d

a + 2d = 9

the fifth term is 21

a5 = a + 4d

a + 4d = 21

a + 2d = 9 (1)

a + 4d = 21 (2)

Subtract (1) from (2) to eliminate a

4d - 2d = 21 - 9

2d = 12

d = 12/2

d = 6

Substitute d = 6 into (1)

a + 2d = 9 (1)

a + 2(6) = 9

a + 12 = 9

a = 9 - 12

a = -3

nth term of this sequence = a + (n - 1)d

= -3 + (n - 1)6

= -3 + 6n - 6

= 6n - 9

an = 6n - 9