Double Integrals in Cylindrical Coordinates: Calculating Areas and Volumes, Study notes of Calculus

Instructions on how to calculate the areas and volumes of various shapes using double integrals in cylindrical coordinates. It includes examples of finding the areas of regions inside and outside of circles and limacons, as well as the volume of solids bounded by paraboloids. The document also includes conversions between polar and rectangular coordinates.

Typology: Study notes

Pre 2010

Uploaded on 08/19/2009

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Double Integrals using Cylindrical Coordinates
Page 1 of 8
Plot the polar function
()
11.5sinr
θ
=+
.
Graph on your graphing utility:
5
2cos2
r
θ
⎛⎞
=+ ⎜⎟
⎝⎠
()
()
cos 3
2cos 4 sin 4
re
θ
θ
θ
⎛⎞
=− +
⎜⎟
⎝⎠
Table 1:
()
11.5sinr
θ
=+
θ
r
0
6
π
3
π
2
π
3
2
π
6
5
π
π
6
7
π
3
4
π
2
3
π
3
5
π
6
11
π
π
2
pf3
pf4
pf5
pf8

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Plot the polar function r = 1 + 1.5sin( θ).

Graph on your graphing utility:

2 cos 2

r

( )

cos 3 2cos 4 sin 4

r e

Table 1: r = 1 +1.5sin( θ)

θ r

Example 1

Find the area of the region inside the circle r = 3sin( θ) and outside the convex limacon

r = 2 − sin ( θ ).

Fact

R

dA ∫∫

finds the (numerical) area of the region R.

Double Integrals in Cylindrical Coordinates ( r , θ, z )

A double integral in cylindrical coordinates always has the form

stop out

start in

r

r

f r r dr d

θ

θ

∫ ∫

where rin = fin ( θ)is the boundary curve closest to the pole, rout = fout ( θ)is the boundary curve

farthest from the pole, the region of integration is swept out (exactly once) in the counterclockwise

direction from θ = θ start to θ = θ stop , and dA = r dr d θ.

Note: If the region of integration includes the pole, then rin = 0.

Example 4

Find the area of the region common to the interiors of the dimpled limacons r = 1.5 − sin( θ )and

r = 1.5 − cos ( θ).

Example 5

Find the volume of the solid bounded by the paraboloids

2 2 z = 4 − xy and

2 2 z = 3 x + 3 y.

Polar to Rectangular Rectangular to Polar

x = cos ( θ) y = sin( θ)

2 2 2 r = x + y

2 2 2

x + y = r tan ( )

y

x

Convert each function to polar form.

x = 7 y = 3

2 2 x + y = y = x

2 2

x + y + x = ( )

2 2 2 2 x x + y = y

0

1

1

0

2 2

2

cos x y dxdy

y

∫ ∫

4 −

0

25

3

x^2 dydx