MTH 142 Practice Problems for Exam
This is a selection of sample problems from sections
11.1 through 11.7.
Last updated April 22, 2004
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- Find all values of the constant
for which f(x) = x2 is
a solution to the differential equation y ' = 3 x + y/x , or
show that there are none.
- Consider the differential equation y' =
- x - y, where is
a positive constant. One (and only one) of the following slope
field plots is definitely not associated with this differential
equation. Which plot does not correspond to the differential
equation? Explain why not. Assume that the lower left corner
corresponds to (0,0).
- Suppose that y(x) is a solution of the initial value
problem y(0.2) = 1, y' = x2 + y , . Use Euler's
to estimate y(0.5). Do all the steps by hand.
- Make a sketch of the slope field for the differential equation
Find an equilibrium solution and determine from the plot if it
is stable or unstable. Explain your answer.
- The number N of certain kind of bacteria in a culture changes
at a rate proportional to N. Initially there were 5 million bacteria,
and 2 hours later there were 7 million.
a) Write down a differential equation satisfied by N.
b) Obtain a formula for N
- A piece of meat is taken out of the freezer and placed on
The temperature of the room is 22 degrees Celsius.
The initial temperature of the meat was -5 degrees
Celsius, and 3 hours later the temperature of the meat is 10
degrees Celsius. Suppose that the temperature T of the meat at
time t satisfies Newton's Law of Heating and Cooling.
a) Write down a differential equation satisfied by T.
b) Find T for any time t.
c) When will the temperature of the meat be equal to 20 degrees
- A large box is sealed except for two pipes for incoming and
outgoing air. The box has volume 10 cubic meters, and it is initially
filled with CO2. Pure oxygen enters the box at a constant rate
of 2 cubic meters per minute. The oxygen mixes with the air in
the box, and the mixture leaves at 2 cubic meters per minute.
Let Q be the amount of oxygen at time t in the box, in cubic meters.
a) write down a differential equation for Q.
b) Find Q at any time t
c) When will be box contain equal parts of oxygen and CO2?
- Solve the initial value problems:
a) , .
b) dr/dx + er = x er
, r = 0 when x = 1.
c) y' + 8 = y ,
y=9 when x = 0.
d) y y' + y y' sin ( y ) = x ,
y=0 when x = 0.
- The growth of certain population is described by the differential
P' = 0.8 P - 0.001 P2
where P(t) is the population at time t days.
The initial population is 25 mill.
a) What is the population at the time when the population is
growing most rapidly?
b) Will the population ever exceed 900 mill.? Explain.
- A radioactive element decomposes so that at any given time,
the rate of change of mass is proportional to the mass present.
a) Write down a differential equation satisfied by M ( t ),
the mass at time t. Solve the differential equation.
b) It takes 10 years for the mass to reduce to 90 % of the original
amount. If currently there are 3 Kg , how long ago was
the mass 5 Kg?
- The following is data corresponding to certain population.
a) Use the data to calculate approximately the blanks
in the table, and plot
b) fit a line as best you can through the points, and then obtain
a differential equation satisfied by .
- The number of individuals y (in millions) of certain
population at time t years satisfies the logistic equation
y' = 20 y - 2 y2.
a) What is the carrying capacity of the population?
b) If the initial population is 5 million, at what rate is the
population changing initially?
c) For what values of the population is the population changing
at a rate of 22 mill/year?