2A.5B A new calculator is purchased for $200; but each year,
it only retains 90% of its value from the year before.
The value of the calculator can be written recursively,
where vo = 200 describes the initial value of the
calculator, vi = 180 represents the value of the calculator
after one year of use, and in describes the value of the
calculator after n years of use. Formulate an equation for
vn describing this situation.