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Mathematics LibreTexts

4.5E: Applications to Curves (Exercises)

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Q4.5.1

In Exercises 4.5.1-4.5.8 find a first order differential equation for the given family of curves.

1. y(x2+y2)=c

2. exy=cy

3. ln|xy|=c(x2+y2)

4. y=x1/2+cx

5. y=ex2+cex2

6. y=x3+cx

7. y=sinx+cex

8. y=ex+c(1+x2)

Q4.5.2

9. Show that the family of circles (xx0)2+y2=1,<x0<,

can be obtained by joining integral curves of two first order differential equations. More specifically, find differential equations for the families of semicircles

(xx0)2+y2=1,x0<x<x0+1,<x0<,

(xx0)2+y2=1,x01<x<x0,<x0<.

10. Suppose f and g are differentiable for all x. Find a differential equation for the family of functions y=f+cg (c=constant).

Q4.5.3

In Exercises 4.5.11-4.5.13 find a first order differential equation for the given family of curves.

11. Lines through a given point (x0,y0).

12. Circles through (1,0) and (1,0).

13. Circles through (0,0) and (0,2).

Q4.5.4

14. Use the method Example 4.5.6 (a) to find the equations of lines through the given points tangent to the parabola y=x2. Also, find the points of tangency.

  1. (5,9)
  2. (6,11)
  3. (6,20)
  4. (3,5)

15.

  1. Show that the equation of the line tangent to the circle x2+y2=1
    at a point (x0,y0) on the circle is y=1x0xy0ifx0±1.
  2. Show that if y is the slope of a nonvertical tangent line to the circle (A) and (x,y) is a point on the tangent line then (y)2(x21)2xyy+y21=0.
  3. Show that the segment of the tangent line (B) on which (xx0)/y0>0 is an integral curve of the differential equation y=xyx2+y21x21,
    while the segment on which (xx0)/y0<0 is an integral curve of the differential equation y=xy+x2+y21x21.
    HINT: Use the quadratic formula to solve (C) for y. Then substitute (B) for y and choose the ± sign in the quadratic formula so that the resulting expression for y reduces to the known slope y=x0/y0
  4. Show that the upper and lower semicircles of (A) are also integral curves of (D) and (E).
  5. Find the equations of two lines through (5,5) tangent to the circle (A), and find the points of tangency.

16.

  1. Show that the equation of the line tangent to the parabola x=y2
    at a point (x0,y0)(0,0) on the parabola is y=y02+x2y0.
  2. Show that if y is the slope of a nonvertical tangent line to the parabola (A) and (x,y) is a point on the tangent line then 4x2(y)24xyy+x=0.
  3. Show that the segment of the tangent line defined in (a) on which x>x0 is an integral curve of the differential equation y=y+y2x2x,
    while the segment on which x<x0 is an integral curve of the differential equation y=yy2x2x,
    HINT: Use the quadratic formula to solve (c) for y. Then substitute (B) for y and choose the ± sign in the quadratic formula so that the resulting expression for y reduces to the known slope of y=12y0
  4. Show that the upper and lower halves of the parabola (A), given by y=x and y=x for x>0, are also integral curves of (D) and (E).

17. Use the results of Exercise 4.5.16 to find the equations of two lines tangent to the parabola x=y2 and passing through the given point. Also find the points of tangency.

  1. (5,2)
  2. (4,0)
  3. (7,4)
  4. (5,3)

18. Find a curve y=y(x) through (1,2) such that the tangent to the curve at any point (x0,y(x0)) intersects the x axis at xI=x0/2.

19. Find all curves y=y(x) such that the tangent to the curve at any point (x0,y(x0)) intersects the x axis at xI=x30.

20. Find all curves y=y(x) such that the tangent to the curve at any point passes through a given point (x1,y1).

21. Find a curve y=y(x) through (1,1) such that the tangent to the curve at any point (x0,y(x0)) intersects the y axis at yI=x30.

22. Find all curves y=y(x) such that the tangent to the curve at any point (x0,y(x0)) intersects the y axis at yI=x0.

23. Find a curve y=y(x) through (0,2) such that the normal to the curve at any point (x0,y(x0)) intersects the x axis at xI=x0+1.

24. Find a curve y=y(x) through (2,1) such that the normal to the curve at any point (x0,y(x0)) intersects the y axis at yI=2y(x0).

Q4.5.5

In Exercises 4.5.25-2.5.29 find the orthogonal trajectories of the given family of curves.

25. x2+2y2=c2

26. x2+4xy+y2=c

27. y=ce2x

28. xyex2=c

29. y=cexx

Q4.5.6

30. Find a curve through (1,3) orthogonal to every parabola of the form y=1+cx2

that it intersects. Which of these parabolas does the desired curve intersect?

31. Show that the orthogonal trajectories of x2+2axy+y2=c

satisfy |yx|a+1|y+x|a1=k.

32. If lines L and L1 intersect at (x0,y0) and α is the smallest angle through which L must be rotated counterclockwise about (x0,y0) to bring it into coincidence with L1, we say that α is the angle from L to L1; thus, 0α<π. If L and L1 are tangents to curves C and C1, respectively, that intersect at (x0,y0), we say that C1 intersects C at the angle α. Use the identity tan(A+B)=tanA+tanB1tanAtanB

to show that if C and C1 are intersecting integral curves of y=f(x,y)andy=f(x,y)+tanα1f(x,y)tanα(απ2),
respectively, then C1 intersects C at the angle α.

33. Use the result of Exercise 4.5.32 to find a family of curves that intersect every nonvertical line through the origin at the angle α=π/4.

34. Use the result of Exercise 4.5.32 to find a family of curves that intersect every circle centered at the origin at a given angle απ/2.


This page titled 4.5E: Applications to Curves (Exercises) is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by William F. Trench via source content that was edited to the style and standards of the LibreTexts platform.

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