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2024cal L2 Note

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Calculus for Management Majors

mathematical thinking through calculus

Jungkai Alfred Chen


Outline

Lecture 2
Feb 22, 2024

I more on calculus of parametric curves


I length
I tangents and area
I quadric surfaces
I sections
I level curves
Length of a Parametric Curve

”Approximate” by Riemann sum of length of line segments!


Length of a Parametric Curve

”Approximate” by Riemann sum of length of line segments!


Z b q
L= dx 2 + dy 2 .
a
Length of a Parametric Curve

Example (Cycloid)
x = r (✓ sin ✓), y = r (1 cos ✓), 0  ✓  2⇡.
Length of a Parametric Curve

Arc length is independent of parametrization.


Length of a Parametric Curve

Since the length (as a function of time) is


Z tr
dx dy
s(t) = ( )2 + ( )2 du.
↵ du du

we have the speed


r
dx 2 dy
0
v (t) = s (t) = ( ) + ( )2 .
du du
Length of a Parametric Curve

Example
x = 2t + 5, y = 10 2t 2 , t 0. Find the speed at the point
(7, 8).
Tangent of Curves

Question
How to determine the tangent of a (parametric) curve?
Tangent of Curves

Given x = f (t), y = g(t), then

dy dy dx
= · .
dt dx dt
Tangent of Curves

Example
Find the slope of the curve x = cos t, y = sin 2t at t = ⇡/2 and
3⇡/2 respectively.
Tangent of Curves

Given x = f (t), y = g(t), then

d 2y
=?
dx 2
Area Associates to a Curve

Example
Find the area under the curve x = cos t, y = sin 2t, 0  t  ⇡/2
and above the x-axis.
Examples of Quadric Surfaces

Example
parabolic cylinder
z = x2
Examples of Quadric Surfaces

Example
Cylinder
x 2 + z 2 = 1 (resp. x 2 + y 2 = 1.).
Examples of Quadric Surfaces

Example
Ellipsoid
x 2 + y 2 /4 + z 2 /9 = 1.
Examples of Quadric Surfaces

Example
(Elliptic) Paraboloid
z = x 2 + y 2 (resp. z = 4x 2 + y 2 ).
Examples of Quadric Surfaces

Example
Hyperbolic Paraboloid
z = x 2 + y 2.
Examples of Quadric Surfaces

Example
Cone
z 2 = x 2 + 4y 2 .
Examples of Quadric Surfaces

Example
Hyperboloid of One Sheet
1 = x 2 + 4y 2 z 2 .
Examples of Quadric Surfaces

Example
Hyperboloid of Two Sheets
1 = x 2 4y 2 + z 2 .

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