** Applications of First Order Equations**

**4.5 APPLICATIONS TO CURVES**

One-Parameter Families of Curves

We begin with two examples of families of curves generated by varying a parameter over a set of real numbers.

Example 4.5.1 For each value of the parameterc, the equation

y−cx2= 0 (4.5.1)

defines a curve in thexy-plane. Ifc 6= 0, the curve is a parabola through the origin, opening upward if c >0or downward ifc <0. Ifc= 0, the curve is thexaxis (Figure4.5.1).

* x*
* y*

Figure 4.5.1 A family of curves defined byy−cx2_{= 0}

Example 4.5.2 For each value of the parametercthe equation

y=x+c (4.5.2)

defines a line with slope 1(Figure4.5.2).

Definition 4.5.1 An equation that can be written in the form

H(x, y, c) = 0 (4.5.3) is said to define aone-parameter family of curvesif,for each value ofcin in some nonempty set of real numbers,the set of points(x, y)that satisfy (4.5.3) forms a curve in thexy-plane.

* x*
* y*

Figure 4.5.2 A family of lines defined byy=x+c

* x*
* y*

Figure 4.5.3 A family of circles defined by

x2_{+}_{y}2_{−}_{c}2_{= 0}

Equations (4.5.1) and (4.5.2) define one–parameter families of curves. (Although (4.5.2) isn’t in the form (4.5.3), it can be written in this form asy−x−c= 0.)

Example 4.5.3 Ifc >0, the graph of the equation

x2+y2−c= 0 (4.5.4)

is a circle with center at(0,0)and radius√c. Ifc = 0, the graph is the single point(0,0). (We don’t regard a single point as a curve.) Ifc < 0, the equation has no graph. Hence, (4.5.4) defines a one– parameter family of curves for positive values ofc. This family consists of all circles centered at(0,0)

(Figure4.5.3).

Example 4.5.4 The equation

x2_{+}_{y}2_{+}_{c}2_{= 0}

does not define a one-parameter family of curves, since no(x, y)satisfies the equation ifc6= 0, and only the single point(0,0)satisfies it ifc= 0.

Recall from Section 1.2 that the graph of a solution of a differential equation is called anintegral curve

of the equation. Solving a first order differential equation usually produces a one–parameter family of integral curves of the equation. Here we are interested in the converse problem: given a one–parameter family of curves, is there a first order differential equation for which every member of the family is an integral curve. This suggests the next definition.

Definition 4.5.2 If every curve in a one-parameter family defined by the equation

H(x, y, c) = 0 (4.5.5) is an integral curve of the first order differential equation

F(x, y, y0) = 0, (4.5.6) then (4.5.6) is said to be adifferential equation for the family.

Section 4.5Applications to Curves 181

To find a differential equation for a one–parameter family we differentiate its defining equation (4.5.5) implicitly with respect tox, to obtain

Hx(x, y, c) +Hy(x, y, c)y0 = 0. (4.5.7)

If this equation doesn’t, then it’s a differential equation for the family. If it does containc, it may be possible to obtain a differential equation for the family by eliminatingcbetween (4.5.5) and (4.5.7).

Example 4.5.5 Find a differential equation for the family of curves defined by

y=cx2. (4.5.8)

Solution Differentiating (4.5.8) with respect toxyields
y0 = 2cx.
Thereforec=y0_{/}_{2}_{x, and substituting this into (4.5.8) yields}

y=xy 0 2

as a differential equation for the family of curves defined by (4.5.8). The graph of any function of the
formy=cx2_{is an integral curve of this equation.}

The next example shows that members of a given family of curves may be obtained by joining integral curves for more than one differential equation.

Example 4.5.6

(a) Try to find a differential equation for the family of lines tangent to the parabolay=x2_{.}

(b) Find two tangent lines to the parabola y = x2 _{that pass through}_{(2}_{,}_{3)}_{, and find the points of}
tangency.

SOLUTION(a) The equation of the line through a given point(x0, y0)with slopemis

y=y0+m(x−x0). (4.5.9)

If(x0, y0)is on the parabola, theny0=x2

0and the slope of the tangent line through (x0, x20)ism= 2x0; hence, (4.5.9) becomes y=x2 0+ 2x0(x−x0), or, equivalently, y=−x2 0+ 2x0x. (4.5.10)

Herex0plays the role of the constantcin Definition4.5.1; that is, varyingx0over(−∞,∞)produces
the family of tangent lines to the parabolay=x2_{.}

Differentiating (4.5.10) with respect toxyieldsy0 _{= 2}_{x0.. We can express}_{x0}_{in terms of}_{x}_{and}_{y}_{by}

rewriting (4.5.10) as

x20−2x0x+y= 0 and using the quadratic formula to obtain

We must choose the plus sign in (4.5.11) ifx < x0and the minus sign ifx > x0; thus,

x0=x+px2_{−}_{y} _{if} _{x < x0}
and

x0=x−px2_{−}_{y} _{if} _{x > x0.}
Sincey0_{= 2}_{x0, this implies that}

y0 _{= 2}_{x}_{+}p

x2_{−}_{y}_{,} _{if} _{x < x0} _{(4.5.12)}

and

y0_{= 2}_{x}

−px2_{−}_{y}_{,} _{if} _{x > x0.} _{(4.5.13)}

Neither (4.5.12) nor (4.5.13) is a differential equation for the family of tangent lines to the parabola
y =x2_{. However, if each tangent line is regarded as consisting of two}_{tangent half lines}_{joined at the}
point of tangency, (4.5.12) is a differential equation for the family of tangent half lines on whichxis less
than the abscissa of the point of tangency (Figure4.5.4(a)), while (4.5.13) is a differential equation for
the family of tangent half lines on whichxis greater than this abscissa (Figure4.5.4(b)). The parabola
y=x2_{is also an integral curve of both (4.5.12) and (4.5.13).}

* y* * y*

* x*
* x*

(a) (b)

Figure 4.5.4

SOLUTION(b) From (4.5.10) the point(x, y) = (2,3)is on the tangent line through(x0, x2

0)if and only if

3 =−x20+ 4x0, which is equivalent to

Section 4.5Applications to Curves 183

Lettingx0= 3in (4.5.10) shows that(2,3)is on the line

y=−9 + 6x, which is tangent to the parabola at(x0, x2

0) = (3,9), as shown in Figure4.5.5 Lettingx0= 1in (4.5.10) shows that(2,3)is on the line

y=−1 + 2x, which is tangent to the parabola at(x0, x2

0) = (1,1), as shown in Figure4.5.5.
* y*
1
2
3
4
5
6
7
8
9
10
11
* x*
1 2 3
* y = x2*
Figure 4.5.5
Geometric Problems

We now consider some geometric problems that can be solved by means of differential equations.

Example 4.5.7 Find curvesy=y(x)such that every point(x0, y(x0))on the curve is the midpoint of the line segment with endpoints on the coordinate axes and tangent to the curve at(x0, y(x0))(Figure4.5.6).

Solution The equation of the line tangent to the curve atP = (x0, y(x0)is
y=y(x0) +y0_{(}_{x0}_{)(}_{x}

−x0).

If we denote thexandyintercepts of the tangent line byxIandyI (Figure4.5.6), then

0 =y(x0) +y0_{(}_{x0}_{)(}_{x}

I −x0) (4.5.14)

and

From Figure4.5.6,P is the midpoint of the line segment connecting(xI,0)and(0, yI)if and only if xI = 2x0andyI = 2y(x0). Substituting the first of these conditions into (4.5.14) or the second into (4.5.15) yields

y(x0) +y0_{(}_{x0}_{)}_{x0}_{= 0}_{.}

Sincex0is arbitrary we drop the subscript and conclude thaty=y(x)satisfies

y+xy0_{= 0}_{,}

which can be rewritten as

(xy)0_{= 0}_{.}

Integrating yieldsxy=c, or

y= c

x.

Ifc = 0this curve is the liney = 0, which does not satisfy the geometric requirements imposed by the problem; thus,c6= 0, and the solutions define a family of hyperbolas (Figure4.5.7).

* x*
* y*
* x*
*I*
* .5 x*
*I*
* y*
*I*
*.5 y*
*I*
Figure 4.5.6
* x*
* y*
Figure 4.5.7

Example 4.5.8 Find curvesy = y(x)such that the tangent line to the curve at any point(x0, y(x0))

intersects thex-axis at(x2

0,0). Figure4.5.8illustrates the situation in the case where the curve is in the first quadrant and0< x <1.

Solution The equation of the line tangent to the curve at(x0, y(x0))is
y=y(x0) +y0_{(}_{x0}_{)(}_{x}

−x0). Since(x2

0,0)is on the tangent line,

0 =y(x0) +y0_{(}_{x0}_{)(}_{x}2
0−x0).

Sincex0is arbitrary we drop the subscript and conclude thaty=y(x)satisfies

y+y0_{(}_{x}2

Section 4.5Applications to Curves 185
* x*
*0*
x_{0}2 * x*
* y*
Figure 4.5.8
* x = 1*
* x*
* y*
Figure 4.5.9
Therefor
y0
y =−
1
x2_{−}_{x}=−
1
x(x−1) =
1
x−
1
x−1,
so
ln|y|= ln|x| −ln|x−1|+k= ln
x
x−1
+k,
and
y= cx
x−1.

Ifc= 0, the graph of this function is thex-axis. Ifc6= 0, it’s a hyperbola with vertical asymptotex= 1

and horizontal asymptotey=c. Figure4.5.9shows the graphs forc6= 0. Orthogonal Trajectories

Two curvesC1andC2are said to beorthogonalat a point of intersection(x0, y0)if they have perpen- dicular tangents at(x0, y0). (Figure4.5.10). A curve is said to be anorthogonal trajectory of a given family of curves if it’s orthogonal to every curve in the family. For example, every line through the origin is an orthogonal trajectory of the family of circles centered at the origin. Conversely, any such circle is an orthogonal trajectory of the family of lines through the origin (Figure4.5.11).

Orthogonal trajectories occur in many physical applications. For example, ifu=u(x, y)is the tem- perature at a point(x, y), the curves defined by

u(x, y) =c (4.5.16)

are calledisothermalcurves. The orthogonal trajectories of this family are calledheat-flowlines, because at any given point the direction of maximum heat flow is perpendicular to the isothermal through the point. Ifurepresents the potential energy of an object moving under a force that depends upon(x, y), the curves (4.5.16) are calledequipotentials, and the orthogonal trajectories are calledlines of force.

From analytic geometry we know that two nonvertical linesL1 andL2 with slopesm1 andm2, re- spectively, are perpendicular if and only ifm2=−1/m1; therefore, the integral curves of the differential equation

y0_{=}

* x*
* y*

Figure 4.5.10 Curves orthogonal at a point of intersection

* x*
* y*

Figure 4.5.11 Orthogonal families of circles and lines

are orthogonal trajectories of the integral curves of the differential equation

y0 _{=}_{f}_{(}_{x, y}_{)}_{,}

because at any point(x0, y0)where curves from the two families intersect the slopes of the respective tangent lines are

m1=f(x0, y0) and m2 =−_{f}_{(}_{x0, y0}1 _{)}.

This suggests a method for finding orthogonal trajectories of a family of integral curves of a first order equation.

Finding Orthogonal Trajectories Step 1.Find a differential equation

y0 _{=}_{f}_{(}_{x, y}_{)}

for the given family.

Step 2.Solve the differential equation

y0_{=}

−_{f}_{(}_{x, y}1 _{)}

to find the orthogonal trajectories.

Example 4.5.9 Find the orthogonal trajectories of the family of circles

x2_{+}_{y}2_{=}_{c}2 _{(}_{c >}_{0)}_{.} _{(4.5.17)}

Solution To find a differential equation for the family of circles we differentiate (4.5.17) implicitly with respect toxto obtain

Section 4.5Applications to Curves 187

or

y0 _{=}

−x

y. Therefore the integral curves of

y0 _{=} y

x

are orthogonal trajectories of the given family. We leave it to you to verify that the general solution of this equation is

y=kx,

wherekis an arbitrary constant. This is the equation of a nonvertical line through(0,0). Theyaxis is also an orthogonal trajectory of the given family. Therefore every line through the origin is an orthogonal trajectory of the given family (4.5.17) (Figure4.5.11). This is consistent with the theorem of plane geometry which states that a diameter of a circle and a tangent line to the circle at the end of the diameter are perpendicular.

Example 4.5.10 Find the orthogonal trajectories of the family of hyperbolas

xy=c (c6= 0) (4.5.18)

(Figure4.5.7).

Solution Differentiating (4.5.18) implicitly with respect toxyields
y+xy0_{= 0}_{,}

or

y0 _{=}

−y_{x};

thus, the integral curves of

y0 _{=}x

y

are orthogonal trajectories of the given family. Separating variables yields

y0y=x and integrating yields

y2−x2=k,

which is the equation of a hyperbola ifk6= 0, or of the linesy=xandy=−xifk= 0(Figure4.5.12). Example 4.5.11 Find the orthogonal trajectories of the family of circles defined by

(x−c)2+y2=c2 (c6= 0). (4.5.19) These circles are centered on thex-axis and tangent to they-axis (Figure4.5.13(a)).

Solution Multiplying out the left side of (4.5.19) yields

* x*
* y*

Figure 4.5.12 Orthogonal trajectories of the hyperbolasxy=c

and differentiating this implicitly with respect toxyields

2(x−c) + 2yy0 _{= 0}_{.}
(4.5.21)
From (4.5.20),
c= x
2_{+}_{y}2
2x ,
so
x−c=x−x
2_{+}_{y}2
2x =
x2_{−}_{y}2
2x .
Substituting this into (4.5.21) and solving fory0_{yields}

y0_{=}y2−x2

2xy . (4.5.22)

The curves defined by (4.5.19) are integral curves of (4.5.22), and the integral curves of

y0 = 2xy

x2_{−}_{y}2

are orthogonal trajectories of the family (4.5.19). This is a homogeneous nonlinear equation, which we studied in Section 2.4. Substitutingy=uxyields

u0x+u= 2x(ux)
x2_{−}_{(}_{ux}_{)}2 =
2u
1−u2,
so
u0_{x}_{=} 2u
1−u2 −u=
u(u2_{+ 1)}
1−u2 ,

Section 4.5Applications to Curves 189

Separating variables yields

1−u2
u(u2_{+ 1)}u
0_{=} 1
x,
or, equivalently,
_{1}
u−
2u
u2_{+ 1}
u0 _{=} 1
x.
Therefore
ln|u| −ln(u2+ 1) = ln|x|+k.
By substitutingu=y/x, we see that

ln|y| −ln|x| −ln(x2+y2) + ln(x2) = ln|x|+k,
which, sinceln(x2_{) = 2 ln}_{|}_{x}_{|}_{, is equivalent to}

ln|y| −ln(x2+y2) =k, or

|y|=ek_{(}_{x}2_{+}_{y}2_{)}_{.}
To see what these curves are we rewrite this equation as

x2+|y|2−e−k

|y|= 0

and complete the square to obtain

x2+ (|y| −e−k_{/}_{2)}2_{= (}_{e}−k_{/}_{2)}2_{.}
This can be rewritten as

x2+ (y−h)2 =h2, where h= e−k 2 ify≥0, −e −k 2 ify≤0.

Thus, the orthogonal trajectories are circles centered on the y axis and tangent to the x axis (Fig- ure 4.5.13(b)). The circles for which h > 0 are above thex-axis, while those for whichh < 0 are below.

* y* * y*

* x*
* x*

(a) (b)

### 4.5 Exercises

In Exercises1–8find a first order differential equation for the given family of curves.

1. y(x2_{+}_{y}2_{) =}_{c} _{2.} _{e}xy_{=}_{cy}
3. ln|xy|=c(x2_{+}_{y}2_{)} _{4.} _{y}_{=}_{x}1/2_{+}_{cx}
5. y=ex2
+ce−x2
6. y=x3+c
x
7. y= sinx+cex
8. y=ex_{+}_{c}_{(1 +}_{x}2_{)}
9. Show that the family of circles

(x−x0)2+y2= 1, −∞< x0<∞,

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

(x−x0)2+y2= 1, x0< x < x0+ 1, −∞< x0<∞,

(x−x0)2+y2= 1, x0−1< x < x0, −∞< x0<∞.

10. Supposefandgare differentiable for allx. Find a differential equation for the family of functions y=f+cg(c=constant).

In Exercises11–13find 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).

14. Use the method Example4.5.6(a)to find the equations of lines through the given points tangent to
the parabolay=x2_{. Also, find the points of tangency.}

(a) (5,9) (b) (6,11) (c) (−6,20) (d) (−3,5)

15. (a) Show that the equation of the line tangent to the circle

x2+y2= 1 (A)

at a point(x0, y0)on the circle is

y= 1−x0x

y0 if x06=±1. (B)

(b) Show that ify0_{is the slope of a nonvertical tangent line to the circle (A) and}_{(}_{x, y}_{)}_{is a point}

on the tangent line then

(y0_{)}2_{(}_{x}2

−1)−2xyy0_{+}_{y}2

Section 4.5Applications to Curves 191

(c) Show that the segment of the tangent line (B) on which(x−x0)/y0>0is an integral curve of the differential equation

y0 _{=}xy−

p

x2_{+}_{y}2_{−}_{1}

x2_{−}_{1} , (D)

while the segment on which(x−x0)/y0<0is an integral curve of the differential equation
y0 _{=}xy+

p

x2_{+}_{y}2_{−}_{1}

x2_{−}_{1} . (E)

HINT:Use the quadratic formula to solve(C)fory0_{. Then substitute}_{(B)}_{for}_{y}_{and choose}

the_{±}sign in the quadratic formula so that the resulting expression fory0 _{reduces to the}

known slopey0_{=}_{−}_{x0/y0}_{.}

(d) Show that the upper and lower semicircles of (A) are also integral curves of (D) and (E). (e) Find the equations of two lines through (5,5) tangent to the circle (A), and find the points of

tangency.

16. (a) Show that the equation of the line tangent to the parabola

x=y2 (A)

at a point(x0, y0)6= (0,0)on the parabola is y=y0

2 +

x

2y0. (B)

(b) Show that ify0 _{is the slope of a nonvertical tangent line to the parabola (A) and}_{(}_{x, y}_{)}_{is a}

point on the tangent line then

4x2(y0_{)}2

−4xyy0_{+}_{x}_{= 0}_{.} _{(C)}

(c) Show that the segment of the tangent line defined in(a)on whichx > x0is an integral curve of the differential equation

y0_{=} y+

p

y2_{−}_{x}

2x , (D)

while the segment on whichx < x0is an integral curve of the differential equation

y0_{=} y−

p

y2_{−}_{x}

2x , (E)

HINT:Use the quadratic formula to solve(C)fory0_{. Then substitute}_{(B)}_{for}_{y}_{and choose}

the_{±}sign in the quadratic formula so that the resulting expression fory0 _{reduces to the}

known slopey0_{=} 1

2y0.

(d) Show that the upper and lower halves of the parabola (A), given byy=√xandy =−√x forx >0, are also integral curves of (D) and (E).

17. Use the results of Exercise16to find the equations of two lines tangent to the parabolax=y2_{and}
passing through the given point. Also find the points of tangency.

(a) (−5,2) (b) (−4,0) (c) (7,4) (d) (5,−3)

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