4.5E: Applications to Curves (Exercises)
( \newcommand{\kernel}{\mathrm{null}\,}\)
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+ce−x2
6. y=x3+cx
7. y=sinx+cex
8. y=ex+c(1+x2)
Q4.5.2
9. Show that the family of circles (x−x0)2+y2=1,−∞<x0<∞,
(x−x0)2+y2=1,x0<x<x0+1,−∞<x0<∞,
(x−x0)2+y2=1,x0−1<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.
- (5,9)
- (6,11)
- (−6,20)
- (−3,5)
15.
- Show that the equation of the line tangent to the circle x2+y2=1at a point (x0,y0) on the circle is y=1−x0xy0ifx0≠±1.
- 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(x2−1)−2xyy′+y2−1=0.
- Show that the segment of the tangent line (B) on which (x−x0)/y0>0 is an integral curve of the differential equation y′=xy−√x2+y2−1x2−1,while the segment on which (x−x0)/y0<0 is an integral curve of the differential equation y′=xy+√x2+y2−1x2−1.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
- Show that the upper and lower semicircles of (A) are also integral curves of (D) and (E).
- Find the equations of two lines through (5,5) tangent to the circle (A), and find the points of tangency.
16.
- Show that the equation of the line tangent to the parabola x=y2at a point (x0,y0)≠(0,0) on the parabola is y=y02+x2y0.
- 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′)2−4xyy′+x=0.
- 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+√y2−x2x,while the segment on which x<x0 is an integral curve of the differential equation y′=y−√y2−x2x,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
- 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.
- (−5,2)
- (−4,0)
- (7,4)
- (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=c226. 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
31. Show that the orthogonal trajectories of x2+2axy+y2=c
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, 0≤α<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+tanB1−tanAtanB
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 α≠π/2.