Chapter 4: Partial Differentiation
Section 4.3: Chain Rule
Example 4.3.20
If the equation z=fx,y,z implicitly defines x=xy,z, obtain xy and xz.
Solution
Mathematical Solution
If z=fx,y,z implicitly defines x=xy,z, then z≡fxy,z,y,z is an identity. Hence, differentiation with respect to y and z via the appropriate forms of the chain rule will result in the following.
0=fx xy+fy ⇒ xy= −fy fx
and
1=fx xz+fz ⇒ zx= 1− fzfx
Maple Solution - Interactive
The following solution, based on just the Context Panel, obtains xy. The modifications necessary for obtaining xz should be obvious.
Obtaining xy via the Context Panel
Write the equation defining xy,z.
Context Panel: Differentiate≻Implicitly Complete dialog as per Figure 4.3.20(a)
z=fx,y,z
z=f⁡x,y,z
→implicit differentiation
−D2⁡f⁡x,y,zD1⁡f⁡x,y,z
Figure 4.3.20(a) Implicit Differentiation dialog
Alternatively, use the Implicit Differentiation task template displayed in Table 4.3.20(a). Make the obvious modifications to obtain xz instead of xy.
Tools≻Tasks≻Browse:
Calculus - Differential≻Derivatives≻Implicit Differentiation≻fx,y,z=gx,y,z
Implicit Differentiation with Three Variables
Enter equation:
Obtain ∂x∂y:
implicitdiff, xy,z,y
Stepwise Calculation:
Replace x with xy,z:
eval,x=xy,z
z=f⁡x⁡y,z,y,z
Apply ∂ ∂y:
diff,y
0=D1⁡f⁡x⁡y,z,y,z⁢∂∂y⁢x⁡y,z+D2⁡f⁡x⁡y,z,y,z
Isolate ∂x∂y:
isolate, diffxy,z,y
∂∂y⁢x⁡y,z=−D2⁡f⁡x⁡y,z,y,zD1⁡f⁡x⁡y,z,y,z
Replace xy,zwith x:
xy=evalrhs,xy,z=x
xy=−D2⁡f⁡x,y,zD1⁡f⁡x,y,z
Table 4.3.20(a) Implicit differentiation task template
It is also possible to implement a solution from first principles via the Context Panel system, as shown below in Table 4.3.20(b).
Write the equation defining xy,z. Press the Enter key.
Context Panel: Evaluate at a Point≻x=xy,z
Context Panel: Solve≻Isolate Expression for≻diff(x(y,z),y) (See Figure 4.3.20(b))
Figure 4.3.20(b) Isolate via Context Panel
f⁡x,y,z=0
→evaluate at point
f⁡x⁡y,z,y,z=0
→differentiate w.r.t. y
D1⁡f⁡x⁡y,z,y,z⁢∂∂y⁢x⁡y,z+D2⁡f⁡x⁡y,z,y,z=0
→isolate for diff(x(y,z),y)
Table 4.3.20(b) Context Panel implementation of implicit derivative via first principles
Maple Solution - Coded
Immediate results via the implicitdiff command
Apply the implicitdiff command.
implicitdiffz=fx,y,z,xy,z,y = −D2⁡f⁡x,y,zD1⁡f⁡x,y,z
implicitdiffz=fx,y,z,xy,z,z = −−1+D3⁡f⁡x,y,zD1⁡f⁡x,y,z
Solution from first principles
Apply the diff and isolate commands to obtain xy.
temp≔diffz=fxy,z,y,z,y
isolatetemp,diffxy,z,y
Apply the diff and isolate commands to obtain xz.
temp≔diffz=fxy,z,y,z,z
1=D1⁡f⁡x⁡y,z,y,z⁢∂∂z⁢x⁡y,z+D3⁡f⁡x⁡y,z,y,z
isolatetemp,diffxy,z,z
∂∂z⁢x⁡y,z=−−1+D3⁡f⁡x⁡y,z,y,zD1⁡f⁡x⁡y,z,y,z
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