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Mathematics LibreTexts

3.7E: Excercises

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Exercise 3.7E.1

In Exercises 1-17 determine which equations are exact and solve them.

1. 6x2y2dx+4x3ydy=0

2. (3ycosx+4xex+2x2ex)dx+(3sinx+3)dy=0

3. 14x2y3dx+21x2y2dy=0

4. (2x2y2)dx+(12y24xy)dy=0

5. (x+y)2dx+(x+y)2dy=0

6. (4x+7y)dx+(3x+4y)dy=0

7. (2y2sinx+3y32x)dx+(4ycosx+9xy2)dy=0

8. (2x+y)dx+(2y+2x)dy=0

9. (3x2+2xy+4y2)dx+(x2+8xy+18y)dy=0

10. (2x2+8xy+y2)dx+(2x2+xy3/3)dy=0

11. (1x+2x)dx+(1y+2y)dy=0

12. (ysinxy+xy2cosxy)dx+(xsinxy+xy2cosxy)dy=0

13. xdx(x2+y2)3/2+ydy(x2+y2)3/2=0

14. (ex(x2y2+2xy2)+6x)dx+(2x2yex+2)dy=0

15. (x2ex2+y(2x2+3)+4x)dx+(x3ex2+y12y2)dy=0

16. (exy(x4y+4x3)+3y)dx+(x5exy+3x)dy=0

17. (3x2cosxyx3ysinxy+4x)dx+(8yx4sinxy)dy=0

Exercise 3.7E.2

In Exercises 18-22 solve the initial value problem.

18. (4x3y26x2y2x3)dx+(2x4y2x3)dy=0,y(1)=3

19. (4ycosx+4sinxcosx+sec2x)dx+(4y4sinx)dy=0,y(π/4)=0

20. (y31)exdx+3y2(ex+1)dy=0,y(0)=0

21. (sinxysinx2cosx)dx+cosxdy=0,y(0)=1

22. (2x1)(y1)dx+(x+2)(x3)dy=0,y(1)=1

Exercise 3.7E.3

23. Solve the exact equation (7x+4y)dx+(4x+3y)dy=0. Plot a direction field and some integral curves for this equation on the rectangle {1x1,1y1}.

24. Solve the exact equation ex(x4y2+4x3y2+1)dx+(2x4yex+2y)dy=0. Plot a direction field and some integral curves for this equation on the rectangle {2x2,1y1}.

25. Plot a direction field and some integral curves for the exact equation (x3y4+x)dx+(x4y3+y)dy=0 on the rectangle {1x1,1y1}. (See Exercise 3.5.37(a)).

26. Plot a direction field and some integral curves for the exact equation (3x2+2y)dx+(2y+2x)dy=0 on the rectangle {2x2,2y2}. (See Exercise 3.5.37(b)).

27.

  1. Solve the exact equation (x3y4+2x)dx+(x4y3+3y)dy=0 implicitly.
  2. For what choices of (x0,y0) does Theorem 2.3.1 imply that the initial value problem (x3y4+2x)dx+(x4y3+3y)dy=0,y(x0)=y0, has a unique solution on an open interval (a,b) that contains x0?
  3. Plot a direction field and some integral curves for (A) on a rectangular region centered at the origin. What is the interval of validity of the solution of (B)?

28.

  1. Solve the exact equation (x2+y2)dx+2xydy=0 implicitly.
  2. For what choices of (x0,y0) does Theorem 2.3.1 imply that the initial value problem (x2+y2)dx+2xydy=0,y(x0)=y0, has a unique solution y=y(x) on some open interval (a,b) that contains x0?
  3. Plot a direction field and some integral curves for (A). From the plot determine, the interval (a,b) of b, the monotonicity properties (if any) of the solution of (B), and limxa+y(x) and limxby(x).

29. Find all functions M such that the equation is exact.

  1. M(x,y)dx+(x2y2)dy=0
  2. M(x,y)dx+2xysinxcosydy=0
  3. M(x,y)dx+(exeysinx)dy=0

30. Find all functions N such that the equation is exact.

  1. (x3y2+2xy+3y2)dx+N(x,y)dy=0
  2. (lnxy+2ysinx)dx+N(x,y)dy=0
  3. (xsinx+ysiny)dx+N(x,y)dy=0

31. Suppose M,N, and their partial derivatives are continuous on an open rectangle R, and G is an antiderivative of M with respect to x; that is, Gx=M. Show that if MyNx in R then the function NGy is not independent of x.

32. Prove: If the equations M1dx+N1dy=0 and M2dx+N2dy=0 are exact on an open rectangle R, so is the equation (M1+M2)dx+(N1+N2)dy=0.

33. Find conditions on the constants A, B, C, and D such that the equation (Ax+By)dx+(Cx+Dy)dy=0 is exact.

34. Find conditions on the constants A, B, C, D, E, and F such that the equation (Ax2+Bxy+Cy2)dx+(Dx2+Exy+Fy2)dy=0 is exact.

35. Suppose M and N are continuous and have continuous partial derivatives My and Nx that satisfy the exactness condition My=Nx on an open rectangle R. Show that if (x,y) is in R and F(x,y)=xx0M(s,y0)ds+yy0N(x,t)dt, then Fx=M and Fy=N.

36. Under the assumptions of Exercise 2.5.35, show that F(x,y)=yy0N(x0,s)ds+xx0M(t,y)dt.

37. Use the method suggested by Exercise 2.5.35, with (x0,y0)=(0,0), to solve the these exact equations:

  1. (x3y4+x)dx+(x4y3+y)dy=0
  2. (x2+y2)dx+2xydy=0
  3. (3x2+2y)dx+(2y+2x)dy=0

38. Solve the initial value problem y+2xy=2xyx2+2x2y+1,y(1)=2.

39. Solve the initial value problem y3xy=2x4(4x33y)3x5+3x3+2y,y(1)=1.

40. Solve the initial value problem y+2xy=ex2(3x+2yex22x+3yex2),y(0)=1.

41. Rewrite the separable equation h(y)y=g(x) as an exact equation M(x,y)dx+N(x,y)dy=0. Show that applying the method of this section to (B) yields the same solutions that would be obtained by applying the method of separation of variables to (A)

42. Suppose all second partial derivatives of M=M(x,y) and N=N(x,y) are continuous and Mdx+Ndy=0 and Ndx+Mdy=0 are exact on an open rectangle R. Show that Mxx+Myy=Nxx+Nyy=0 on R.

43. Suppose all second partial derivatives of F=F(x,y) are continuous and Fxx+Fyy=0 on an open rectangle R. (A function with these properties is said to be harmonic; see also Exercise 2.5.42.) Show that Fydx+Fxdy=0 is exact on R, and therefore there’s a function G such that Gx=Fy and Gy=Fx in R. (A function G with this property is said to be a harmonic conjugate of F.)

44. Verify that the following functions are harmonic, and find all their harmonic conjugates. (See Exercise 3.5.43.)

  1. x2y2
  2. excosy
  3. x33xy2
  4. cosxcoshy
  5. sinxcoshy

3.7E: Excercises is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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