# Elementary differential equations 7th edition - Boyce W.E

ISBN 0-471-31999-6

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PROBLEMS 1. Consider a tank used in certain hydrodynamic experiments. After one experiment the tank

I contains 200 liters of a dye solution with a concentration of 1 g/liter. To prepare for the

next experiment, the tank is to be rinsed with fresh water flowing in at a rate of 2 liters/min, the well-stirred solution flowing out at the same rate. Find the time that will elapse before the concentration of dye in the tank reaches 1% of its original value.

2. A tank initially contains 120 liters of pure water. A mixture containing a concentration of Y g/liter of salt enters the tank at a rate of 2 liters/min, and the well-stirred mixture leaves the tank at the same rate. Find an expression in terms of y for the amount of salt in the tank at any time t. Also find the limiting amount of salt in the tank as t ^ <x.

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Chapter 2. First Order Differential Equations

3. A tank originally contains 100 gal of fresh water. Then water containing 1 lb of salt per gallon is poured into the tank at a rate of 2 gal/min, and the mixture is allowed to leave at the same rate. After 10 min the process is stopped, and fresh water is poured into the tank at a rate of 2 gal/min, with the mixture again leaving at the same rate. Find the amount of salt in the tank at the end of an additional 10 min.

4. A tank with a capacity of 500 gal originally contains 200 gal of water with 100 lb of salt in solution. Water containing 1 lb of salt per gallon is entering at a rate of 3 gal/min, and the mixture is allowed to flow out of the tank at a rate of 2 gal/min. Find the amount of salt in the tank at any time prior to the instant when the solution begins to overflow. Find the concentration (in pounds per gallon) of salt in the tank when it is on the point of overflowing. Compare this concentration with the theoretical limiting concentration if the tank had infinite capacity.

? 5. A tank contains 100 gallons of water and 50 oz of salt. Water containing a salt concentration

of 4 (1 + 2 sin t) oz/gal flows into the tank at a rate of 2 gal/min, and the mixture in the tank flows out at the same rate.

(a) Find the amount of salt in the tank at any time.

(b) Plot the solution for a time period long enough so that you see the ultimate behavior of the graph.

(c) The long-time behavior of the solution is an oscillation about a certain constant level. What is this level? What is the amplitude of the oscillation?

6. Suppose that a sum S0 is invested at an annual rate of return r compounded continuously.

(a) Find the time T required for the original sum to double in value as a function of r.

(b) Determine T if r = 7%.

(c) Find the return rate that must be achieved if the initial investment is to double in 8 years.

7. A young person with no initial capital invests k dollars per year at an annual rate of return r. Assume that investments are made continuously and that the return is compounded continuously.

(a) Determine the sum S(t) accumulated at any time t.

(b) If r = 7.5%, determine k so that $1 million will be available for retirement in 40 years.

(c) If k = $2000/year, determine the return rate r that must be obtained to have $1 million available in 40 years.

? 8. Person A opens an IRA at age 25, contributes $2000/year for 10 years, but makes no addi-

tional contributions thereafter. Person B waits until age 35 to open an IRA and contributes $2000/year for 30 years. There is no initial investment in either case.

(a) Assuming a return rate of 8%, what is the balance in each IRA at age 65?

(b) For a constant, but unspecified, return rate r, determine the balance in each IRA at age 65 as a function of r.

(c) Plot the difference in the balances from part (b) for 0 < r < 0.10.

(d) Determine the return rate for which the two IRA’s have equal value at age 65.

9. A certain college graduate borrows $8000 to buy a car. The lender charges interest at an annual rate of 10%. Assuming that interest is compounded continuously and that the borrower makes payments continuously at a constant annual rate k, determine the payment rate k that is required to pay off the loan in 3 years. Also determine how much interest is paid during the 3-year period.

10. A home buyer can afford to spend no more than $800/month on mortgage payments. Suppose that the interest rate is 9% and that the term of the mortgage is 20 years. Assume that interest is compounded continuously and that payments are also made continuously.

(a) Determine the maximum amount that this buyer can afford to borrow.

(b) Determine the total interest paid during the term of the mortgage.

11. How are the answers to Problem 10 changed if the term of the mortgage is 30 years?

2.3 Modeling with First Order Equations

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? 12. A recent college graduate borrows $100,000 at an interest rate of 9% to purchase a con-

dominium. Anticipating steady salary increases, the buyer expects to make payments at a monthly rate of 800(1 + t /120), where t is the number of months since the loan was made.

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