Question

A 4 = 9 and  a 7 = 1 8
Find a1 and d for an arithmetic sequence with these terms

Answer 1

`a_1=0`

d=3

Explanation:

The arithmetic sequence has the fourth term to be 9.

`a_4=9`

`a_1+3d=9`....(1)

The seventh term is 18

`a_7=18`

`a_1+6d=18`....(2)

Subtract equation (1) from equation (2).

`6d-3d=18-9`

3d=9

d=3

Put d=3 in equation 1 and solve for
`a_1`

`a_1+3(3)=9`

`a_1+9=9`

`a_1=9-9`

`a_1=0`