Answer:
[tex]a_1=0[/tex]
d=3
Step-by-step explanation:
The arithmetic sequence has the fourth term to be 9.
[tex]a_4=9[/tex]
[tex]a_1+3d=9[/tex]....(1)
The seventh term is 18
[tex]a_7=18[/tex]
[tex]a_1+6d=18[/tex]....(2)
Subtract equation (1) from equation (2).
[tex]6d-3d=18-9[/tex]
3d=9
d=3
Put d=3 in equation 1 and solve for [tex]a_1[/tex]
[tex]a_1+3(3)=9[/tex]
[tex]a_1+9=9[/tex]
[tex]a_1=9-9[/tex]
[tex]a_1=0[/tex]
