Robin bought a computer for $1,250. It will depreciate, or decrease in value, by 10% each year that she owns it. a) Is the sequence formed by the value at the beginning of each year arithmetic, geometric, or neither? Explain. b) Write an explicit formula to represent the sequence. c) Find the value of the computer at the beginning of the 6th year.

Remember that the general formula of geometric sequence is [tex]a _{n} =(a_{1})(r^{n-1})[/tex]
where 
[tex]a_{n}[/tex] is the nth term
[tex]a_{1}[/tex] is the difference
[tex]n[/tex] is the place of the term in the sequence
Also, to find [tex]r[/tex] we will use the formula: [tex]r= \frac{a_{n}}{a_{n-1}} [/tex]
where 
[tex]a_{n}[/tex] is the current term in the sequence 
[tex]a_{n-1}[/tex] is the previous term 

a) Lets find the three first terms of our sequence to check what type of sequence we have:
We know for our problem that the initial value of the computer is $1250, so our first term is 1250. In other words [tex]a_{1}=1250[/tex].
To fin our second term [tex]a_{2][/tex], we are going to subtract 10% of the value to our original value:
[tex](1250)( \frac{10}{100} )=125[/tex] and [tex]1250-125=1125[/tex], so 
[tex]a_{2}=1125[/tex]
To find our third term [tex]a_{3}[/tex] we are going to subtract yet again 10% to our current value:
[tex](1125)( \frac{10}{100} )=112.5[/tex] and [tex]1125-112.5=1012.5[/tex], so
[tex]a_{3}=1012.5[/tex]
Now that we have our sequence [tex]1250 , 1125 , 1012.5[/tex] lets check if we have a consistent [tex]r[/tex] to prove we have a geometric sequence:
- with [tex]a_{n}=1012.5[/tex], and [tex]a_{n-1}=1125[/tex]:
  [tex]r= \frac{1012.5}{1125}=0.9[/tex]
- with [tex]a_{n}=1125[/tex], and [tex]a_{n-1}=1250[/tex]:
  [tex]r= \frac{1125}{1250}=0.9[/tex]
Look! our [tex]r[/tex]s are the same, so we can conclude that we have a geometric sequence.

b) To do this we just need to replece the values of our sequence in the general formula of a geometric sequence. We know from our previous point that [tex]a_{1}=1250[/tex] and [tex]r=0.9[/tex]. So lets replace those values in geometric sequence formula to find our explicit formula:
[tex]a_{n}=1250(0.9)^{n-1}[/tex]

c) To find the value of the computer at the beginning of the 6th year, we just need to find the 6th therm in our geometric sequence:
[tex]a_{n}=1250(0.9)^{n-1}[/tex]
[tex]a_{6}=1250(0.9)^{6-1}[/tex]
[tex]a_{6}=1250(0.9)^{5}[/tex]
[tex]a_{6}=738.1125[/tex]
We can conclude that the value of the computer at the beginning of the 6th year will be $738.1125