# calculus

Suppose that the sales at Borders bookstores went from 68 million dollars in 1990 to 427 million dollars in 1995 . Find an exponential function to model the sales (in millions of dollars) as a function of years,t, since 1990.

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1. t = 0 in 1990
t = 5 in 1995

s = A e^kt
when t = 0, s = 68
so
68 = A

when t = 5
427 = 68 e^5k
6.28 = e^5k
ln 6.28 = 1.84 = 5 k
so
k = .367
so
s = 68 e^.367 t

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