Chapter 2: Space Curves
Section 2.4: Curvature
Example 2.4.2
Show that the circle x−h2+y−k2=r2 everywhere has constant curvature, that is, show κ=1/r.
Solution
Mathematical Solution
Implicitly differentiate the equation of the circle to obtain y′:
x−h2+y−k2
=r2
2x−h+2y−ky′
=0
y′
=−x−hy−k
Obtain and simplify the denominator of κ:
1+y′23/2
=1+−x−hy−k23/2
=y−k2+x−h2y−k23/2
=r2y−k23/2
=r3y−k3
The final simplification hinges on the positivity of r. Note also that the square root of y−k2 is y−k, which matters because y−k can be both positive and negative along the circle.
Implicitly differentiate y′ to obtain y″, the numerator of κ:
y″
=ddx−x−hy−k
=y−k−x−hy′y−k2
=y−k−x−h−x−hy−ky−k2
=y−k2+x−h2y−k3
=r2y−k3
Finally, obtain κ=|y″|1+y′3/2=r2y−k3r3|y−k|3=1r.
Maple Solution - Interactive
Initialize
Tools≻Load Package: Student Precalculus
Loading Student:-Precalculus
Obtain y′
Control-drag the equation of the circle and press the Enter key.
Context Panel: Differentiate≻Implicitly In the dialog that appears (see Figure 2.4.2(a)), set y as the dependent variable and x as the independent variable. Write x in the "Differentiate with respect to" box.
Figure 2.4.2(a)
x−h2+y−k2=r2→assign to a nameC
C
x−h2+y−k2=r2
→implicit differentiation
−x+hy−k
Obtain y″
Reference the equation of the circle by its equation label and press the Enter key.
Context Panel: Differentiate≻Implicitly Write x,x in the "Differentiate with respect to" box. (See Figure 2.4.2(a).)
−h2−2⁢h⁢x+k2−2⁢k⁢y+x2+y2−k3+3⁢k2⁢y−3⁢k⁢y2+y3
Form and simplify κ=|y″|1+y′23/2
Using equation labels, write the expression for κ, then press the Enter key.
Context Panel: Simplify≻Assuming Real
Context Panel: Student Precalculus≻Complete the Square≻Designated Quantities≻x,y
Context Panel: Simplify≻With Side Relations≻C
Context Panel: Simplify≻Assuming Positive
1+23/2
h2−2⁢h⁢x+k2−2⁢k⁢y+x2+y2−k3+3⁢k2⁢y−3⁢k⁢y2+y31+−x+h2y−k23/2
→assuming real
h2−2⁢h⁢x+k2−2⁢k⁢y+x2+y2h2−2⁢h⁢x+k2−2⁢k⁢y+x2+y23/2
→complete square
x−h2+y−k2x−h2+y−k23/2
= simplify siderels
r2r23/2
→assuming positive
1r
Maple Solution - Coded
Define the circle.
C≔x−h2+y−k2=r2:
Apply the implicitdiff and simplify commands.
K≔simplifyabsimplicitdiffC,y,x,x1+implicitdiffC,y,x23/2 assuming positive
Apply the CompleteSquare command from the Student Precalculus package.
K1≔Student:-Precalculus:-CompleteSquareK,x,y
Apply the simplify command with C as a side relation.
K2≔simplifyK1,C
Apply the simplify command.
simplifyK2 assuming r>0
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