We have
a2 = 4
a3 = 2
a4 = 1
calculate the
difference
a3 - a2 = -2
a4 - a3 = -1
The differences
are not equal---------- >the sequence is not arithmetic.
Getting the
ratio:
a3/a2 = 0.5
a4/a3 = 0.5
The common ratio
is 0.5. Using the general form for a geometric series:
an = a1 r^(n-1)
for n = 2----------------
> a2=4
4 = a1 (0.5)^(2-1)
a1 = 8
therefore
an = 8 (1/2)^(n-1)
the answer is an = 8 (1/2)^(n-1)
the answer is none of those indicated in the question
