** First Order Equations**

**2.4 TRANSFORMATION OF NONLINEAR EQUATIONS INTO SEPARABLE EQUATIONS**

In Section 2.1 we found that the solutions of a linear nonhomogeneous equation

Section 2.4Transformation of Nonlinear Equations into Separable Equations 63

are of the formy=uy1, wherey1is a nontrivial solution of the complementary equation

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

(2.4.1)

anduis a solution of

u0_{y1}_{(}_{x}_{) =}_{f}_{(}_{x}_{)}_{.}

Note that this last equation is separable, since it can be rewritten as

u0_{=} f(x)

y1(x).

In this section we’ll consider nonlinear differential equations that are not separable to begin with, but can be solved in a similar fashion by writing their solutions in the formy=uy1, wherey1is a suitably chosen known function andusatisfies a separable equation. We’llsay in this case that wetransformedthe given equation into a separable equation.

Bernoulli Equations

ABernoulli equationis an equation of the form

y0_{+}_{p}_{(}_{x}_{)}_{y}_{=}_{f}_{(}_{x}_{)}_{y}r_{,} _{(2.4.2)}

wherer can be any real number other than0or1. (Note that (2.4.2) is linear if and only ifr = 0or r= 1.) We can transform (2.4.2) into a separable equation by variation of parameters: ify1is a nontrivial solution of (2.4.1), substitutingy=uy1into (2.4.2) yields

u0_{y1}_{+}_{u}_{(}_{y}0

1+p(x)y1) =f(x)(uy1)r, which is equivalent to the separable equation

u0_{y1}_{(}_{x}_{) =}_{f}_{(}_{x}_{) (}_{y1}_{(}_{x}_{))}r
ur _{or} u0
ur =f(x) (y1(x))
r−1
,
sincey0
1+p(x)y1= 0.

Example 2.4.1 Solve the Bernoulli equation y0

−y=xy2. (2.4.3)

Solution Sincey1=ex_{is a solution of}_{y}0_{−}_{y} _{= 0}_{, we look for solutions of (2.4.3) in the form}_{y}_{=}_{ue}x_{,}
where

u0_{e}x_{=}_{xu}2_{e}2x _{or, equivalently,} _{u}0_{=}_{xu}2_{e}x_{.}
Separating variables yields

u0

u2 =xe
x_{,}
and integrating yields

−1_{u}= (x−1)ex+c.
Hence,

u=−_{(}_{x} 1
−1)ex_{+}_{c}

−2 −1.5 −1 −0.5 0 0.5 1 1.5 2
−2
−1.5
−1
−0.5
0
0.5
1
1.5
2
* x*
* y*

Figure 2.4.1 A direction field and integral curves fory0

−y=xy2

and

y=−_{x} 1
−1 +ce−x.

Figure2.4.1shows direction field and some integral curves of (2.4.3). Other Nonlinear Equations That Can be Transformed Into Separable Equations

We’ve seen that the nonlinear Bernoulli equation can be transformed into a separable equation by the substitutiony = uy1ify1 is suitably chosen. Now let’s discover a sufficient condition for a nonlinear first order differential equation

y0 _{=}_{f}_{(}_{x, y}_{)} _{(2.4.4)}

to be transformable into a separable equation in the same way. Substitutingy=uy1into (2.4.4) yields
u0_{y1}_{(}_{x}_{) +}_{uy}0
1(x) =f(x, uy1(x)),
which is equivalent to
u0_{y1}_{(}_{x}_{) =}_{f}_{(}_{x, uy1}_{(}_{x}_{))}
−uy0
1(x). (2.4.5)
If
f(x, uy1(x)) =q(u)y0
1(x)
for some functionq, then (2.4.5) becomes

u0_{y1}_{(}_{x}_{) = (}_{q}_{(}_{u}_{)}

−u)y0

1(x), (2.4.6)

which is separable. After checking for constant solutionsu≡u0such thatq(u0) =u0, we can separate variables to obtain u0 q(u)−u = y0 1(x) y1(x).

Section 2.4Transformation of Nonlinear Equations into Separable Equations 65

Homogeneous Nonlinear Equations

In the text we’ll consider only the most widely studied class of equations for which the method of the preceding paragraph works. Other types of equations appear in Exercises44–51.

The differential equation (2.4.4) is said to behomogeneousifxandy occur inf in such a way that f(x, y)depends only on the ratioy/x; that is, (2.4.4) can be written as

y0_{=}_{q}_{(}_{y/x}_{)}_{,} _{(2.4.7)}

whereq=q(u)is a function of a single variable. For example,

y0_{=} y+xe−y/x
x =
y
x+e
−y/x
and
y0 _{=}y2+xy−x2
x2 =
y
x
2
+y
x−1
are of the form (2.4.7), with

q(u) =u+e−u _{and} _{q}_{(}_{u}_{) =}_{u}2_{+}_{u}

−1,

respectively. The general method discussed above can be applied to (2.4.7) withy1 =x(and therefore y0

1= 1). Thus, substitutingy=uxin (2.4.7) yields

u0_{x}_{+}_{u}_{=}_{q}_{(}_{u}_{)}_{,}

and separation of variables (after checking for constant solutionsu≡u0such thatq(u0) =u0) yields u0

q(u)−u =

1

x.

Before turning to examples, we point out something that you may’ve have already noticed: the defini- tion ofhomogeneous equationgiven here isn’t the same as the definition given in Section 2.1, where we said that a linear equation of the form

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

is homogeneous. We make no apology for this inconsistency, since we didn’t create it historically,homo-

geneoushas been used in these two inconsistent ways. The one having to do with linear equations is the

most important. This is the only section of the book where the meaning defined here will apply.

Sincey/xis in general undefined ifx = 0, we’ll consider solutions of nonhomogeneous equations only on open intervals that do not contain the pointx= 0.

Example 2.4.2 Solve

y0 _{=}y+xe
−y/x

x . (2.4.8)

Solution Substitutingy=uxinto (2.4.8) yields

u0_{x}_{+}_{u}_{=}ux+xe−ux/x

x =u+e

−u_{.}

Simplifying and separating variables yields

euu0 _{=} 1

Integrating yieldseu_{= ln}_{|}_{x}_{|}_{+}_{c. Therefore}_{u}_{= ln(ln}_{|}_{x}_{|}_{+}_{c}_{)}_{and}_{y}_{=}_{ux}_{=}_{x}_{ln(ln}_{|}_{x}_{|}_{+}_{c}_{)}_{.}
Figure2.4.2shows a direction field and integral curves for (2.4.8).

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
−2
−1.5
−1
−0.5
0
0.5
1
1.5
2
* x*
* y*

Figure 2.4.2 A direction field and some integral curves fory0_{=} y+xe
−y/x
x
Example 2.4.3
(a) Solve
x2y0_{=}_{y}2_{+}_{xy}
−x2. (2.4.9)

(b) Solve the initial value problem

x2y0 _{=}_{y}2_{+}_{xy}

−x2, y(1) = 2. (2.4.10)

SOLUTION(a) We first find solutions of (2.4.9) on open intervals that don’t contain x = 0. We can

rewrite (2.4.9) as

y0 _{=}y2+xy−x2

x2 forxin any such interval. Substitutingy=uxyields

u0_{x}_{+}_{u}_{=}(ux)
2_{+}_{x}_{(}_{ux}_{)}
−x2
x2 =u
2_{+}_{u}
−1,
so
u0x=u2−1. (2.4.11)

Section 2.4Transformation of Nonlinear Equations into Separable Equations 67

By inspection this equation has the constant solutionsu≡1andu≡ −1. Thereforey=xandy=−x
are solutions of (2.4.9). Ifuis a solution of (2.4.11) that doesn’t assume the values_{±}1on some interval,
separating variables yields

u0

u2_{−}_{1} =

1

x, or, after a partial fraction expansion,

1
2
1
u−1 −
1
u+ 1
u0_{=} 1
x.
Multiplying by 2 and integrating yields

ln
u−1
u+ 1
= 2 ln|x|+k,
or
u−1
u+ 1
=ek_{x}2_{,}
which holds if
u−1
u+ 1 =cx
2 _{(2.4.12)}

wherecis an arbitrary constant. Solving foruyields

u=1 +cx
2
1−cx2.
−2 −1.5 −1 −0.5 0 0.5 1 1.5 2
−2
−1.5
−1
−0.5
0
0.5
1
1.5
2
* y*
* x*

Figure 2.4.3 A direction field and integral curves for

x2_{y}0 _{=}_{y}2_{+}_{xy}_{−}_{x}2
* x*
* y*
1
2
Figure 2.4.4 Solutions ofx2_{y}0 _{=}_{y}2_{+}_{xy}_{−}_{x}2_{,}
y(1) = 2
Therefore
y=ux=x(1 +cx
2_{)}
1−cx2 (2.4.13)

is a solution of (2.4.10) for any choice of the constantc. Settingc = 0 in (2.4.13) yields the solution y =x. However, the solutiony =−xcan’t be obtained from (2.4.13). Thus, the solutions of (2.4.9) on intervals that don’t containx= 0arey =−xand functions of the form (2.4.13).

The situation is more complicated ifx= 0is the open interval. First, note thaty=−xsatisfies (2.4.9) on(−∞,∞). Ifc1andc2are arbitrary constants, the function

y=
x(1 +c1x2_{)}
1−c1x2 , a < x <0,
x(1 +c2x2_{)}
1−c2x2 , 0≤x < b,
(2.4.14)
is a solution of (2.4.9) on(a, b), where
a=
−√1_{c1} ifc1>0,
−∞ ifc1≤0,
and b=
1
√_{c2} ifc2>0,
∞ ifc2≤0.

We leave it to you to verify this. To do so, note that ifyis any function of the form (2.4.13) theny(0) = 0

andy0_{(0) = 1}_{.}

Figure2.4.3shows a direction field and some integral curves for (2.4.9).

SOLUTION(b) We could obtaincby imposing the initial conditiony(1) = 2in (2.4.13), and then solving

forc. However, it’s easier to use (2.4.12). Sinceu = y/x, the initial conditiony(1) = 2implies that u(1) = 2. Substituting this into (2.4.12) yieldsc= 1/3. Hence, the solution of (2.4.10) is

y= x(1 +x

2_{/}_{3)}

1−x2_{/}_{3} .

The interval of validity of this solution is(−√3,√3). However, the largest interval on which (2.4.10) has a unique solution is(0,√3). To see this, note from (2.4.14) that any function of the form

y=
x(1 +cx2_{)}
1−cx2 , a < x≤0,
x(1 +x2_{/}_{3)}
1−x2_{/}_{3} , 0≤x <
√
3,
(2.4.15)

is a solution of (2.4.10) on(a,√3), wherea=−1/√cifc >0ora=−∞ifc≤0. (Why doesn’t this contradict Theorem2.3.1?)

Figure2.4.4shows several solutions of the initial value problem (2.4.10). Note that these solutions coincide on(0,√3).

In the last two examples we were able to solve the given equations explicitly. However, this isn’t always possible, as you’ll see in the exercises.

### 2.4 Exercises

In Exercises1–4solve the given Bernoulli equation.

1. y0_{+}_{y}_{=}_{y}2 2. 7xy0
−2y=−x
2
y6
3. x2_{y}0_{+ 2}_{y}_{= 2}_{e}1/x_{y}1/2 _{4.} _{(1 +}_{x}2_{)}_{y}0_{+ 2}_{xy}_{=} 1
(1 +x2_{)}_{y}

In Exercises 5 and6 find all solutions. Also, plot a direction field and some integral curves on the

Section 2.4Transformation of Nonlinear Equations into Separable Equations 69

5. C/G y0_{−}_{xy}_{=}_{x}3_{y}3_{;} _{{−}_{3}_{≤}_{x}_{≤}_{3}_{,}_{2}_{≤}_{y}_{≥}_{2}_{}}
6. C/G y0

−1 +_{3}_{x}xy=y4; {−2≤x≤2,−2≤y≤2}

In Exercises7–11solve the initial value problem.

7. y0
−2y=xy3_{,} _{y}_{(0) = 2}√_{2}
8. y0_{−}_{xy}_{=}_{xy}3/2_{,} _{y}_{(1) = 4}
9. xy0_{+}_{y}_{=}_{x}4_{y}4_{,} _{y}_{(1) = 1}_{/}_{2}
10. y0_{−}_{2}_{y}_{= 2}_{y}1/2_{,} _{y}_{(0) = 1}
11. y0
−4y=48x
y2 , y(0) = 1

In Exercises12and13solve the initial value problem and graph the solution.

12. C/G x2_{y}0_{+ 2}_{xy}_{=}_{y}3_{,} _{y}_{(1) = 1}_{/}√_{2}
13. C/G y0

−y=xy1/2_{,} _{y}_{(0) = 4}

14. You may have noticed that the logistic equation
P0 _{=}_{aP}_{(1}

−αP)

from Verhulst’s model for population growth can be written in Bernoulli form as

P0

−aP =−aαP2_{.}

This isn’t particularly interesting, since the logistic equation is separable, and therefore solvable by the method studied in Section 2.2. So let’s consider a more complicated model, whereais a positive constant andαis a positive continuous function ofton[0,∞). The equation for this model is

P0

−aP=−aα(t)P2, a non-separable Bernoulli equation.

(a) Assuming thatP(0) =P0>0, findP fort >0. HINT:Express your result in terms of the
integralR_{0}tα(τ)eaτ_{dτ}_{.}

(b) Verify that your result reduces to the known results for the Malthusian model whereα= 0, and the Verhulst model whereαis a nonzero constant.

(c) Assuming that lim t→∞e −atZ t 0 α(τ)eaτdτ =L exists (finite or infinite), findlimt→∞P(t).

In Exercises15–18solve the equation explicitly.

15. y0_{=} y+x
x 16. y
0_{=} y
2_{+ 2}_{xy}
x2
17. xy3_{y}0_{=}_{y}4_{+}_{x}4
18. y0_{=} y
x+ sec
y
x

In Exercises19-21solve the equation explicitly. Also, plot a direction field and some integral curves on the indicated rectangular region.

19. C/G x2_{y}0 _{=}_{xy}_{+}_{x}2_{+}_{y}2_{;} _{{−}_{8}_{≤}_{x}_{≤}_{8}_{,}_{−}_{8}_{≤}_{y}_{≤}_{8}_{}}
20. C/G xyy0 _{=}_{x}2_{+ 2}_{y}2_{;} _{{−}_{4}_{≤}_{x}_{≤}_{4}_{,}_{−}_{4}_{≤}_{y}_{≤}_{4}_{}}
21. C/G y0_{=} 2y

2_{+}_{x}2_{e}−(y/x)2

2xy ; {−8≤x≤8,−8≤y≤8}

In Exercises22–27solve the initial value problem.

22. y0_{=} xy+y
2
x2 , y(−1) = 2
23. y0_{=} x
3_{+}_{y}3
xy2 , y(1) = 3
24. xyy0_{+}_{x}2_{+}_{y}2_{= 0}_{,} _{y}_{(1) = 2}
25. y0_{=} y
2_{−}_{3}_{xy}_{−}_{5}_{x}2
x2 , y(1) =−1
26. x2_{y}0_{= 2}_{x}2_{+}_{y}2_{+ 4}_{xy,} _{y}_{(1) = 1}
27. xyy0 _{= 3}_{x}2_{+ 4}_{y}2_{,} _{y}_{(1) =}√_{3}

In Exercises28–34solve the given homogeneous equation implicitly.

28. y0_{=} x+y
x−y 29. (y
0_{x}_{−}_{y}_{)(ln}_{|}_{y}_{| −}_{ln}_{|}_{x}_{|}_{) =}_{x}
30. y0_{=} y
3_{+ 2}_{xy}2_{+}_{x}2_{y}_{+}_{x}3
x(y+x)2 31. y
0_{=} x+ 2y
2x+y
32. y0_{=} y
y−2x 33. y0= xy
2_{+ 2}_{y}3
x3_{+}_{x}2_{y}_{+}_{xy}2
34. y0_{=} x
3_{+}_{x}2_{y}_{+ 3}_{y}3
x3_{+ 3}_{xy}2
35. L

(a) Find a solution of the initial value problem

x2_{y}0 _{=}_{y}2_{+}_{xy}_{−}_{4}_{x}2_{,} _{y}_{(}_{−}_{1) = 0} _{(A)}
on the interval(−∞,0). Verify that this solution is actually valid on(−∞,∞).

(b) Use Theorem2.3.1to show that (A) has a unique solution on(−∞,0). (c) Plot a direction field for the differential equation in (A) on a square

{−r≤x≤r,−r≤y≤r},

whereris any positive number. Graph the solution you obtained in(a)on this field. (d) Graph other solutions of (A) that are defined on(−∞,∞).

Section 2.4Transformation of Nonlinear Equations into Separable Equations 71

(e) Graph other solutions of (A) that are defined only on intervals of the form(−∞, a), where is a finite positive number.

36. L

(a) Solve the equation

xyy0 _{=}_{x}2_{−}_{xy}_{+}_{y}2 _{(A)}

implicitly.

(b) Plot a direction field for (A) on a square

{0≤x≤r,0≤y≤r}

whereris any positive number.

(c) LetKbe a positive integer. (You may have to try several choices forK.) Graph solutions of the initial value problems

xyy0 _{=}_{x}2

−xy+y2, y(r/2) = kr

K,

fork= 1,2, . . . ,K. Based on your observations, find conditions on the positive numbersx0 andy0such that the initial value problem

xyy0 _{=}_{x}2

−xy+y2, y(x0) =y0, (B)

has a unique solution (i) on(0,∞)or (ii) only on an interval(a,∞), wherea >0?

(d) What can you say about the graph of the solution of (B) asx→ ∞? (Again, assume that x0>0andy0>0.)

37. L

(a) Solve the equation

y0 =2y

2_{−}_{xy}_{+ 2}_{x}2

xy+ 2x2 (A)

implicitly.

(b) Plot a direction field for (A) on a square

{−r≤x≤r,−r≤y≤r}

wherer is any positive number. By graphing solutions of (A), determine necessary and sufficient conditions on(x0, y0)such that (A) has a solution on (i)(−∞,0)or (ii)(0,∞)

such thaty(x0) =y0.

38. L Follow the instructions of Exercise37for the equation

y0= xy+x

2_{+}_{y}2

xy .

39. L Pick any nonlinear homogeneous equationy0 _{=}_{q}_{(}_{y/x}_{)}_{you like, and plot direction fields on}

the square_{{−}r≤x≤r, −r≤y ≤r}, wherer >0. What happens to the direction field as you
varyr? Why?

40. Prove: Ifad−bc6= 0, the equation

y0_{=} ax+by+α

cx+dy+β can be transformed into the homogeneous nonlinear equation

dY dX =

aX+bY cX+dY

by the substitutionx=X−X0, y=Y −Y0, whereX0andY0are suitably chosen constants.

In Exercises41-43use a method suggested by Exercise40to solve the given equation implicitly.

41. y0_{=} −6x+y−3
2x−y−1 42. y
0_{=} 2x+y+ 1
x+ 2y−4
43. y0_{=} −x+ 3y−14
x+y−2

In Exercises44–51find a functiony1 such that the substitutiony =uy1transforms the given equation

into a separable equation of the form(2.4.6). Then solve the given equation explicitly.

44. 3xy2_{y}0 _{=}_{y}3_{+}_{x} _{45.} _{xyy}0 _{= 3}_{x}6_{+ 6}_{y}2
46. x3_{y}0_{= 2(}_{y}2_{+}_{x}2_{y}_{−}_{x}4_{)} _{47.} _{y}0_{=}_{y}2_{e}−x_{+ 4}_{y}_{+ 2}_{e}x
48. y0_{=} y
2_{+}_{y}_{tan}_{x}_{+ tan}2_{x}
sin2x
49. x(lnx)2_{y}0 _{=}_{−}_{4(ln}_{x}_{)}2_{+}_{y}_{ln}_{x}_{+}_{y}2
50. 2x(y+ 2√x)y0_{= (}_{y}_{+}√_{x}_{)}2
51. (y+ex2
)y0_{= 2}_{x}_{(}_{y}2_{+}_{ye}x2
+e2x2
)

52. Solve the initial value problem

y0+ 2

xy=

3x2_{y}2_{+ 6}_{xy}_{+ 2}

x2_{(2}_{xy}_{+ 3)} , y(2) = 2.
53. Solve the initial value problem

y0_{+} 3

xy=

3x4_{y}2_{+ 10}_{x}2_{y}_{+ 6}

x3_{(2}_{x}2_{y}_{+ 5)} , y(1) = 1.

54. Prove: Ifyis a solution of a homogeneous nonlinear equationy0 _{=}_{q}_{(}_{y/x}_{)}_{, so is}_{y1} _{=}_{y}_{(}_{ax}_{)}_{/a,}

whereais any nonzero constant.

55. Ageneralized Riccati equationis of the form

y0 =P(x) +Q(x)y+R(x)y2. (A)

(IfR≡ −1, (A) is aRiccati equation.) Lety1be a known solution andyan arbitrary solution of (A). Letz=y−y1. Show thatzis a solution of a Bernoulli equation withn= 2.

Section 2.5Exact Equations 73

In Exercises56–59, given thaty1is a solution of the given equation, use the method suggested by Exercise

55to find other solutions.

56. y0_{= 1 +}_{x}_{−}_{(1 + 2}_{x}_{)}_{y}_{+}_{xy}2_{;} _{y1}_{= 1}
57. y0_{=}_{e}2x_{+ (1}_{−}_{2}_{e}x_{)}_{y}_{+}_{y}2_{;} _{y1}_{=}_{e}x
58. xy0 _{= 2}
−x+ (2x−2)y−xy2_{;} _{y1}_{= 1}
59. xy0 _{=}_{x}3_{+ (1}_{−}_{2}_{x}2_{)}_{y}_{+}_{xy}2_{;} _{y1}_{=}_{x}
2.5 EXACT EQUATIONS

In this section it’s convenient to write first order differential equations in the form

M(x, y)dx+N(x, y)dy= 0. (2.5.1)

This equation can be interpreted as

M(x, y) +N(x, y)dy

dx = 0, (2.5.2)

wherexis the independent variable andyis the dependent variable, or as

M(x, y)dx

dy +N(x, y) = 0, (2.5.3)

wherey is the independent variable andxis the dependent variable. Since the solutions of (2.5.2) and (2.5.3) will often have to be left in implicit, form we’ll say thatF(x, y) = cis an implicit solution of (2.5.1) if every differentiable functiony = y(x)that satisfiesF(x, y) = cis a solution of (2.5.2) and every differentiable functionx=x(y)that satisfiesF(x, y) =cis a solution of (2.5.3).

Here are some examples:

Equation (2.5.1) Equation (2.5.2) Equation (2.5.3)

3x2_{y}2_{dx}_{+ 2}_{x}3_{y dy}_{= 0} _{3}_{x}2_{y}2_{+ 2}_{x}3_{y} dy
dx = 0 3x
2_{y}2 dx
dy + 2x
3_{y}_{= 0}
(x2_{+}_{y}2_{)}_{dx}_{+ 2}_{xy dy}_{= 0} _{(}_{x}2_{+}_{y}2_{) + 2}_{xy}dy
dx = 0 (x
2_{+}_{y}2_{)}dx
dy + 2xy= 0

3ysinx dx−2xycosx dy= 0 3ysinx−2xycosxdy

dx = 0 3ysinx dx

dy −2xycosx= 0 Note that a separable equation can be written as (2.5.1) as

M(x)dx+N(y)dy= 0.

We’ll develop a method for solving (2.5.1) under appropriate assumptions onM andN. This method is an extension of the method of separation of variables (Exercise41). Before stating it we consider an example.

Example 2.5.1 Show that

x4y3+x2y5+ 2xy=c (2.5.4)

is an implicit solution of

(4x3y3+ 2xy5+ 2y)dx+ (3x4y2+ 5x2y4+ 2x)dy= 0. (2.5.5)

Solution Regardingyas a function ofxand differentiating (2.5.4) implicitly with respect toxyields

(4x3y3+ 2xy5+ 2y) + (3x4y2+ 5x2y4+ 2x)dy

dx = 0.

Similarly, regardingxas a function ofyand differentiating (2.5.4) implicitly with respect toyyields

(4x3_{y}3_{+ 2}_{xy}5_{+ 2}_{y}_{)}dx
dy + (3x

4_{y}2_{+ 5}_{x}2_{y}4_{+ 2}_{x}_{) = 0}_{.}

Therefore (2.5.4) is an implicit solution of (2.5.5) in either of its two possible interpretations.

You may think this example is pointless, since concocting a differential equation that has a given implicit solution isn’t particularly interesting. However, it illustrates the next important theorem, which we’ll prove by using implicit differentiation, as in Example2.5.1.

Theorem 2.5.1 IfF =F(x, y)has continuous partial derivativesFxandFy, then

F(x, y) =c (c=constant), (2.5.6)

is an implicit solution of the differential equation

Fx(x, y)dx+Fy(x, y)dy= 0. (2.5.7)

Proof Regardingyas a function ofxand differentiating (2.5.6) implicitly with respect toxyields Fx(x, y) +Fy(x, y)

dy dx = 0.

On the other hand, regardingxas a function ofyand differentiating (2.5.6) implicitly with respect toy yields

Fx(x, y)dx

dy+Fy(x, y) = 0.

Thus, (2.5.6) is an implicit solution of (2.5.7) in either of its two possible interpretations. We’ll say that the equation

M(x, y)dx+N(x, y)dy= 0 (2.5.8)

isexacton an an open rectangleRif there’s a functionF =F(x, y)suchFxandFyare continuous, and

Fx(x, y) =M(x, y) and Fy(x, y) =N(x, y) (2.5.9)

for all(x, y)inR. This usage of “exact” is related to its usage in calculus, where the expression

Fx(x, y)dx+Fy(x, y)dy

(obtained by substituting (2.5.9) into the left side of (2.5.8)) is theexact differential of F.

Example2.5.1shows that it’s easy to solve (2.5.8) if it’s exactandwe know a functionF that satisfies (2.5.9). The important questions are:

Section 2.5Exact Equations 75

QUESTION1. Given an equation (2.5.8), how can we determine whether it’s exact? QUESTION2. If (2.5.8) is exact, how do we find a functionF satisfying (2.5.9)?

To discover the answer to Question 1, assume that there’s a functionF that satisfies (2.5.9) on some open rectangleR, and in addition thatF has continuous mixed partial derivativesFxyandFyx. Then a theorem from calculus implies that

Fxy=Fyx. (2.5.10)

IfFx=M andFy=N, differentiating the first of these equations with respect toyand the second with respect toxyields

Fxy=My and Fyx=Nx. (2.5.11)

From (2.5.10) and (2.5.11), we conclude that a necessary condition for exactness is thatMy=Nx. This motivates the next theorem, which we state without proof.

Theorem 2.5.2 [The Exactness Condition] SupposeM andN are continuous and have continuous par-

tial derivativesMyandNxon an open rectangleR.Then

M(x, y)dx+N(x, y)dy= 0

is exact onRif and only if

My(x, y) =Nx(x, y) (2.5.12)

for all(x, y)inR..

To help you remember the exactness condition, observe that the coefficients ofdxanddyare differ- entiated in (2.5.12) with respect to the “opposite” variables; that is, the coefficient ofdxis differentiated with respect toy, while the coefficient ofdyis differentiated with respect tox.

Example 2.5.2 Show that the equation

3x2_{y dx}_{+ 4}_{x}3_{dy}_{= 0}
is not exact on any open rectangle.

Solution Here

M(x, y) = 3x2y and N(x, y) = 4x3 so

My(x, y) = 3x2 and Nx(x, y) = 12x2.

ThereforeMy =Nxon the linex= 0, but not on any open rectangle, so there’s no functionF such that Fx(x, y) =M(x, y)andFy(x, y) =N(x, y)for all(x, y)on any open rectangle.

The next example illustrates two possible methods for finding a functionF that satisfies the condition Fx=M andFy=NifM dx+N dy= 0is exact.

Example 2.5.3 Solve

(4x3_{y}3_{+ 3}_{x}2_{)}_{dx}_{+ (3}_{x}4_{y}2_{+ 6}_{y}2_{)}_{dy}_{= 0}_{.} _{(2.5.13)}

Solution (Method 1) Here

and

My(x, y) =Nx(x, y) = 12x3y2

for all(x, y). Therefore Theorem2.5.2implies that there’s a functionF such that

Fx(x, y) =M(x, y) = 4x3y3+ 3x2 (2.5.14)

and

Fy(x, y) =N(x, y) = 3x4y2+ 6y2 (2.5.15) for all(x, y). To findF, we integrate (2.5.14) with respect toxto obtain

F(x, y) =x4y3+x3+φ(y), (2.5.16)

whereφ(y)is the “constant” of integration. (Hereφis “constant” in that it’s independent ofx, the variable of integration.) Ifφis any differentiable function ofythenF satisfies (2.5.14). To determineφso thatF