In Exercises
21
−
25
, solving the differential equations that arise from modeling may require using integration by parts. [See formula
(
1
)
].
A Savings Account with Withdrawals After depositing an initial amount of
$
10
,
000
in a savings account that earns
4
%
interest compounded continuously, a person continued to make deposits for a certain period of time and then started to make withdrawals from the account. The annual rate of deposits was given by
3000
−
500
t
dollars per year,
t
years from the time the account was opened. (Here, negative rates of deposits correspond to withdrawals.)
a. How many years did the person contribute to the account before starting to withdraw money from it?
b. Let
P
(
t
)
denote the amount of money in the account,
t
years after the initial deposit. Find an initial-value problem satisfied by
P
(
t
)
. (Assume that the deposits and withdrawals were made continuously.