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Mathematics LibreTexts

8.R: Chapter 8 Review Exercises

  • Gilbert Strang & Edwin “Jed” Herman
  • OpenStax

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True or False? Justify your answer with a proof or a counterexample.

1) The differential equation y=3x2ycos(x)y is linear.

2) The differential equation y=xy is separable.

Answer
F

3) You can explicitly solve all first-order differential equations by separation or by the method of integrating factors.

4) You can determine the behavior of all first-order differential equations using directional fields or Euler’s method.

Answer
T

For the following problems, find the general solution to the differential equations.

5) y=x2+3ex2x

6) y=2x+cos1x

Answer
y(x)=2xln(2)+xcos1x1x2+C

7) y=y(x2+1)

8) y=eysinx

Answer
y(x)=ln(Ccosx)

9) y=3x2y

10) y=ylny

Answer
y(x)=eeC+x

For the following problems, find the solution to the initial value problem.

11) y=8xlnx3x4,y(1)=5

12) y=3xcosx+2,y(0)=4

Answer
y(x)=4+32x2+2xsinx

13) xy=y(x2),y(1)=3

14) y=3y2(x+cosx),y(0)=2

Answer
y(x)=21+3(x2+2sinx)

15) (x1)y=y2,y(0)=0

16) y=3yx+6x2,y(0)=1

Answer
y(x)=2x22x1323e3x

For the following problems, draw the directional field associated with the differential equation, then solve the differential equation. Draw a sample solution on the directional field.

17) y=2yy2

18) y=1x+lnxy, for x>0

Answer

y(x)=Cex+lnx

A direction field with arrows pointing up and to the right along a logarithmic curve that approaches negative infinity as x goes to zero and increases as x goes to infinity.

For the following problems, use Euler’s Method with n=5 steps over the interval t=[0,1]. Then solve the initial-value problem exactly. How close is your Euler’s Method estimate?

19) y=4yx,y(0)=1

20) y=3x2y,y(0)=0

Answer
Euler: 0.6939,
Exact solution: y(x)=3xe2x2+ln(3)

For the following problems, set up and solve the differential equations.

21) A car drives along a freeway, accelerating according to a=5\sin(πt), where t represents time in minutes. Find the velocity at any time t, assuming the car starts with an initial speed of 60 mph.

22) You throw a ball of mass 2 kilograms into the air with an upward velocity of 8 m/s. Find exactly the time the ball will remain in the air, assuming that gravity is given by g=9.8\,\text{m/s}^2.

Answer
\frac{40}{49} second

23) You drop a ball with a mass of 5 kilograms out an airplane window at a height of 5000 m. How long does it take for the ball to reach the ground?

24) You drop the same ball of mass 5 kilograms out of the same airplane window at the same height, except this time you assume a drag force proportional to the ball’s velocity, using a proportionality constant of 3 and the ball reaches terminal velocity. Solve for the distance fallen as a function of time. How long does it take the ball to reach the ground?

Answer
x(t)=5000+\frac{245}{9}−\frac{49}{3}t−\frac{245}{9}e^{−5/3t}, \quad t=307.8 seconds

25) A drug is administered to a patient every 24 hours and is cleared at a rate proportional to the amount of drug left in the body, with proportionality constant 0.2. If the patient needs a baseline level of 5 mg to be in the bloodstream at all times, how large should the dose be?

26) A 1000-liter tank contains pure water and a solution of 0.2 kg salt/L is pumped into the tank at a rate of 1 L/min and is drained at the same rate. Solve for total amount of salt in the tank at time t.

Answer
T(t)=200\left(1−e^{−t/1000}\right)

27) You boil water to make tea. When you pour the water into your teapot, the temperature is 100°C. After 5 minutes in your 15°C room, the temperature of the tea is 85°C. Solve the equation to determine the temperatures of the tea at time t. How long must you wait until the tea is at a drinkable temperature (72°C)?

28) The human population (in thousands) of Nevada in 1950 was roughly 160. If the carrying capacity is estimated at 10 million individuals, and assuming a growth rate of 2\% per year, develop a logistic growth model and solve for the population in Nevada at any time (use 1950 as time = 0). What population does your model predict for 2000? How close is your prediction to the true value of 1,998,257?

Answer
P(t)=\dfrac{1600000e^{0.02t}}{9840+160e^{0.02t}}

29) Repeat the previous problem but use Gompertz growth model. Which is more accurate?


This page titled 8.R: Chapter 8 Review Exercises is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Gilbert Strang & Edwin “Jed” Herman (OpenStax) via source content that was edited to the style and standards of the LibreTexts platform.

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