The formular for a sequence with first term, a, and increases at a rate of d is given by:
[tex]a_n=a+(n-1)d[/tex]
Here: a = 1200, d = 900
Thus,
[tex]a_n=1200+900(n-1) \\ \\ =1200+900n-90=300+900n[/tex]
Therefore, the function, written in sequence notation, which models Sarah’s savings account balance n years after starting her new job is an = 300 + 900n