Indicate the general rule for this sequence. Find a 1 and the common difference. a 4 = 18 a 7 = 9

• LammettHash: If [tex]d[/tex] is the common difference in an arithmetic sequence, whose [tex]n[/tex]-th term is [tex]a_n[/tex], then
  
  [tex]a_7=a_6+d[/tex]
  [tex]a_6=a_5+d[/tex]
  [tex]a_5=a_4+d[/tex]
  
  [tex]\implies a_7=a_4+3d[/tex]
  
  We have
  
  [tex]9=18+3d\implies3d=-9\implies d=-3[/tex]