Step-by-step explanation:
an arithmetic sequence means that every new term is the old term plus a constant number.
in our case that constant is +50.
and our starting value a0 = 250
so, the first 4 terms are
$250, $300, $350, $400
for the function formula we know
a0 = $250
a1 = a0 + 50 = 250 + 50 = $300
a2 = a1 + 50 = a0 + 50 + 50 = a0 + 2×50 = $350
a3 = a2 + 50 = a0 + 3×50 = $400
so, we see
an = a0 + n×50 = 250 + 50n
therefore,
f(n) = 250 + 50n
after 9 months saving we reached a9, meaning n=9.
f(9) = 250 + 50×9 = 250 + 450 = $700
