Section 9.4 Areas and Lengths in Polar Coordinates 2010 Kiryl Tsishchanka
Areas and Lengths in Polar Coordinates
In this section we develop the formula for the area of a region whose boundary is given by a polar equation.We need to use the formula for the area of a sector of a circle
A =1
2r2θ (1)
where r is the radius and θ is the radian measure of the central angle. Formula 1 follows from the factthat the area of a sector is proportional to its central angle:
A =θ
2π· πr2 =
1
2r2θ
Let R be the region bounded by the polar curve r = f(θ) and by the rays θ = a and θ = b, where f is apositive continuous function and where 0 < b − a ≤ 2π.
We divide the interval [a, b] into subintervals with endpoints θ0, θ1, θ2, . . . , θn and equal width ∆θ. Therays θ = θi then divide R into n smaller regions with central angle ∆θ = θi − θi−1. If we choose θ∗i in theith subinterval [θi−1, θi], then the area ∆Ai of the ith region is approximated by the area of the sector ofa circle with central angle ∆θ and radius f(θ∗i ). Thus from Formula 1 we have
∆Ai ≈1
2[f(θ∗i )]
2∆θ (2)
and so an approximation to the total area A of R is A ≈n
∑
i=1
1
2[f(θ∗i )]
2∆θ. One can see that the approxima-
tion in (2) improves as n → ∞. But the sums in (2) are Riemann sums for the function g(θ) =1
2[f(θ)]2,
so
limn→∞
n∑
i=1
1
2[f(θ∗i )]
2∆θ =
∫ b
a
1
2[f(θ)]2dθ
It therefore appears plausible (and can in fact be proved) that the formula for the area A of the polarregion R is
A =
∫ b
a
1
2[f(θ)]2dθ (3)
This formula is often written as
A =
∫ b
a
1
2r2dθ (4)
1
Section 9.4 Areas and Lengths in Polar Coordinates 2010 Kiryl Tsishchanka
EXAMPLE: Find the area of each of the following regions:
(a) (b) (c) (d)
Solution:
(a) We have
A =
∫ 3π/4
π/4
1
2· 12dθ =
1
2
∫ 3π/4
π/4
dθ =1
2
(
3π
4− π
4
)
=1
2
(
2π
4
)
=π
4
(b) We have
A =
∫ 2π+π/4
3π/4
1
2· 12dθ =
1
2
∫ 2π+π/4
3π/4
dθ =1
2
(
2π +π
4− 3π
4
)
=1
2
(
2π − 2π
4
)
= π − π
4=
3π
4
(c) We have
A =
∫ 7π/4
5π/4
1
2· 12dθ =
1
2
∫ 7π/4
5π/4
dθ =1
2
(
7π
4− 5π
4
)
=1
2
(
2π
4
)
=π
4
(d) We have
A =
∫ 2π+π/4
7π/4
1
2· 12dθ =
1
2
∫ 2π+π/4
7π/4
dθ =1
2
(
2π +π
4− 7π
4
)
=1
2
(
2π − 6π
4
)
= π − 3π
4=
π
4
or
A =
∫ π/4
−π/4
1
2· 12dθ =
1
2
∫ π/4
−π/4
dθ =1
2
(π
4−
(
−π
4
))
=1
2
(π
4+
π
4
)
=π
4
EXAMPLE: Find the area of the inner loop of r = 2 + 4 cos θ.
2
Section 9.4 Areas and Lengths in Polar Coordinates 2010 Kiryl Tsishchanka
EXAMPLE: Find the area of the inner loop of r = 2 + 4 cos θ.
Solution: We first find a and b:
2 + 4 cos θ = 0 =⇒ cos θ = −1
2=⇒ θ =
2π
3,
4π
3
Therefore the area is
A =
∫ 4π/3
2π/3
1
2(2 + 4 cos θ)2dθ =
∫ 4π/3
2π/3
1
2(4 + 16 cos θ + 16 cos2 θ)dθ
=
∫ 4π/3
2π/3
(2 + 8 cos θ + 8 cos2 θ)dθ =
∫ 4π/3
2π/3
(
2 + 8 cos θ + 8 · 1 + cos 2θ
2
)
dθ
=
∫ 4π/3
2π/3
(2 + 8 cos θ + 4(1 + cos 2θ)dθ =
∫ 4π/3
2π/3
(6 + 8 cos θ + 4 cos 2θ)dθ
=[
6θ + 8 sin θ + 2 sin 2θ]4π/3
2π/3= 4π − 6
√3 ≈ 2.174
EXAMPLE: Find the area enclosed by one loop of the four-leaved rose r = cos 2θ.
3
Section 9.4 Areas and Lengths in Polar Coordinates 2010 Kiryl Tsishchanka
EXAMPLE: Find the area enclosed by one loop of the four-leaved rose r = cos 2θ.
Solution: Notice that the region enclosed by the right loop is swept out by a ray that rotates from θ = −π/4to θ = π/4. Therefore, Formula 4 gives
A =
∫ π/4
−π/4
1
2r2dθ =
1
2
∫ π/4
−π/4
cos2 2θdθ =
∫ π/4
0
cos2 2θdθ
=
∫ π/4
0
1
2(1 + cos 4θ)dθ =
1
2
[
θ +1
4sin 4θ
]π/4
0
=π
8
Let R be the region bounded by curves with polar equations r = f(θ), r = g(θ), θ = a, and θ = b, wheref(θ) ≥ g(θ) ≥ 0 and 0 < b − a ≤ 2π. Then the area A of R is
A =
∫ b
a
1
2
(
[f(θ)]2 − [g(θ)]2)
dθ
EXAMPLE: Find the area that lies inside r = 3 + 2 sin θ and outside r = 2.
4
Section 9.4 Areas and Lengths in Polar Coordinates 2010 Kiryl Tsishchanka
EXAMPLE: Find the area that lies inside r = 3 + 2 sin θ and outside r = 2.
Solution: We first find a and b:
3 + 2 sin θ = 2 =⇒ sin θ = −1
2=⇒ θ =
7π
6, −π
6
(
11π
6
)
Therefore the area is
A =
∫ 7π/6
−π/6
1
2
[
(3 + 2 sin θ)2 − 22]
dθ =
∫ 7π/6
−π/6
1
2(5 + 12 sin θ + 4 sin2 θ)dθ
=
∫ 7π/6
−π/6
1
2
(
5 + 12 sin θ + 4 · 1 − cos 2θ
2
)
dθ =
∫ 7π/6
−π/6
1
2(5 + 12 sin θ + 2(1 − cos 2θ))dθ
=
∫ 7π/6
−π/6
1
2(7 + 12 sin θ − 2 cos 2θ)dθ =
1
2
[
7θ − 12 cos θ − sin 2θ]7π/6
−π/6
=11√
3
2+
14π
3≈ 24.187
EXAMPLE: Find the area of the region outside r = 3 + 2 sin θ and inside r = 2.
5
Section 9.4 Areas and Lengths in Polar Coordinates 2010 Kiryl Tsishchanka
EXAMPLE: Find the area of the region outside r = 3 + 2 sin θ and inside r = 2.
Solution: We have
A =
∫ 11π/6
7π/6
1
2
[
22 − (3 + 2 sin θ)2]
dθ
=
∫ 11π/6
7π/6
1
2(−5 − 12 sin θ − 4 sin2 θ)dθ
=
∫ 11π/6
7π/6
1
2(−7 − 12 sin θ + 2 cos 2θ)dθ =
1
2
[
− 7θ + 12 cos θ + sin 2θ]11π/6
7π/6=
11√
3
2− 7π
3≈ 2.196
EXAMPLE: Find all points of intersection of the curves r = cos 2θ and r =1
2.
Solution: If we solve the equations r = cos 2θ and r =1
2, we get cos 2θ =
1
2and, therefore,
2θ = π/3, 5π/3, 7π/3, 11π/3
Thus the values of θ between 0 and 2π that satisfy both equations are
θ = π/6, 5π/6, 7π/6, 11π/6
We have found four points of intersection:(
1
2, π/6
)
,
(
1
2, 5π/6
)
,
(
1
2, 7π/6
)
, and
(
1
2, 11π/6
)
However, you can see from the above figure that the curves have four other points of intersection — namely,(
1
2, π/3
)
,
(
1
2, 2π/3
)
,
(
1
2, 4π/3
)
, and
(
1
2, 5π/3
)
These can be found using symmetry or by noticing that another equation of the circle is r = −1
2and then
solving the equations r = cos 2θ and r = −1
2.
6
Section 9.4 Areas and Lengths in Polar Coordinates 2010 Kiryl Tsishchanka
Arc Length
To find the length of a polar curve r = f(θ), a ≤ θ ≤ b, we regard θ as a parameter and write theparametric equations of the curve as
x = r cos θ = f(θ) cos θ y = r sin θ = f(θ) sin θ
Using the Product Rule and differentiating with respect to θ, we obtain
dx
dθ=
dr
dθcos θ − r sin θ
dy
dθ=
dr
dθsin θ + r cos θ
So, using cos2 θ + sin2 θ = 1, we have
(
dx
dθ
)2
+
(
dy
dθ
)2
=
(
dr
dθ
)2
cos2 θ − 2rdr
dθcos θ sin θ + r2 sin2 θ
+
(
dr
dθ
)2
sin2 θ + 2rdr
dθsin θ cos θ + r2 cos2 θ =
(
dr
dθ
)2
+ r2
Assuming that f ′ is continuous, we can use one of the formulas from Section 9.2 to write the arc length as
L =
∫ b
a
√
(
dx
dθ
)2
+
(
dy
dθ
)2
dθ
Therefore, the length of a curve with polar equation r = f(θ), a ≤ θ ≤ b, is
L =
∫ b
a
√
r2 +
(
dr
dθ
)2
dθ (5)
EXAMPLE: Find the length of the curve r = θ, 0 ≤ θ ≤ 1.
7
Section 9.4 Areas and Lengths in Polar Coordinates 2010 Kiryl Tsishchanka
EXAMPLE: Find the length of the curve r = θ, 0 ≤ θ ≤ 1.
Solution: We have
L =
∫ 1
0
√θ2 + 1dθ =
θ = tanx =⇒√
θ2 + 1 =√
tan2 x + 1 =√
sec2 x = | sec x| = sec x
dθ = d tanx
dθ = sec2 xdx
=
∫ π/4
0
sec3 xdx =
[
1
2(sec x tan x + ln | sec x + tanx|)
]π/4
0
=1
2(√
2 + ln(1 +√
2))
EXAMPLE: Find the length of the cardioid r = 1 − cos θ.
8
Section 9.4 Areas and Lengths in Polar Coordinates 2010 Kiryl Tsishchanka
EXAMPLE: Find the length of the cardioid r = 1 − cos θ.
Solution: The full length of the cardioid is given by the parameter interval 0 ≤ θ ≤ 2π, so Formula 5 gives
L =
∫ 2π
0
√
r2 +
(
dr
dθ
)2
dθ =
∫ 2π
0
√
(1 − cos θ)2 + sin2 θdθ
=
∫ 2π
0
√
1 − 2 cos θ + cos2 θ + sin2 θdθ
=
∫ 2π
0
√2 − 2 cos θdθ
=
∫ 2π
0
√
4 sin2 θ
2dθ
=
∫ 2π
0
2
∣
∣
∣
∣
sinθ
2
∣
∣
∣
∣
dθ
=
∫ 2π
0
2 sinθ
2dθ
= −4 cosθ
2
]2π
0
= 4 + 4 = 8
EXAMPLE: Find the length of the cardioid r = 1 + sin θ.
9
Section 9.4 Areas and Lengths in Polar Coordinates 2010 Kiryl Tsishchanka
EXAMPLE: Find the length of the cardioid r = 1 + sin θ.
Solution 1: Note thatr = 1 + sin θ = 1 − cos
(
θ +π
2
)
Therefore the graph of r = 1 + sin θ is the rotation of the graph of r = 1 − cos θ. Hence the length of thecardioid r = 1 + sin θ is 8 by the previous Example.
Solution 2: The full length of the cardioid is given by the parameter interval 0 ≤ θ ≤ 2π, so Formula 5gives
L =
∫ 2π
0
√
r2 +
(
dr
dθ
)2
dθ
=
∫ 2π
0
√
(1 + sin θ)2 + cos2 θdθ
=
∫ 2π
0
√
1 + 2 sin θ + sin2 θ + cos2 θdθ
=
∫ 2π
0
√2 + 2 sin θdθ
=
∫ 2π
0
√
2 − 2 cos(
θ +π
2
)
dθ =
θ +π
2= u
d(
θ +π
2
)
= du
dθ = du
=
∫ 5π/2
π/2
√2 − 2 cos udu
=
∫ 5π/2
π/2
√
4 sin2 u
2du =
∫ 5π/2
π/2
2∣
∣
∣sin
u
2
∣
∣
∣du =
∫ 2π
π/2
2∣
∣
∣sin
u
2
∣
∣
∣du +
∫ 5π/2
2π
2∣
∣
∣sin
u
2
∣
∣
∣du
=
∫ 2π
π/2
2 sinu
2du −
∫ 5π/2
2π
2 sinu
2du
= −4 cosu
2
]2π
π/2+ 4 cos
u
2
]5π/2
2π
=(
−4 cos π + 4 cosπ
4
)
+
(
4 cos5π
4− 4 cos π
)
= (4 + 2√
2) + (−2√
2 + 4) = 8
10
Section 9.4 Areas and Lengths in Polar Coordinates 2010 Kiryl Tsishchanka
Solution 3: The full length of the cardioid is given by the parameter interval 0 ≤ θ ≤ 2π, so Formula 5gives
L =
∫ 2π
0
√
r2 +
(
dr
dθ
)2
dθ =
∫ 2π
0
√
(1 + sin θ)2 + cos2 θdθ =
∫ 2π
0
√
1 + 2 sin θ + sin2 θ + cos2 θdθ
=
∫ 2π
0
√2 + 2 sin θdθ =
√2
∫ 2π
0
√1 + sin θdθ =
√2
∫ 2π
0
√1 + sin θ
√1 − sin θ√
1 − sin θdθ
=√
2
∫ 2π
0
√
1 − sin2 θ√1 − sin θ
dθ =√
2
∫ 2π
0
√cos2 θ√
1 − sin θdθ =
√2
∫ 2π
0
| cos θ|√1 − sin θ
dθ
=√
2
∫ π/2
0
cos θ√1 − sin θ
dθ −√
2
∫ 3π/2
π/2
cos θ√1 − sin θ
dθ +√
2
∫ 2π
3π/2
cos θ√1 − sin θ
dθ
Note that
∫
cos θ√1 − sin θ
dθ =
1 − sin θ = u
d(1 − sin θ) = du
− cos θdθ = du
cos θdθ = −du
= −∫
du√u
= −∫
u−1/2du = − u−1/2+1
−1/2 + 1+ C
= −2√
u + C
= −2√
1 − sin θ + C
Therefore
L = −2√
2√
1 − sin θ]π/2
0+ 2
√2√
1 − sin θ]3π/2
π/2− 2
√2√
1 − sin θ]2π
3π/2
= −2√
2(
√
1 − sin(π/2) −√
1 − sin 0)
+2√
2(
√
1 − sin(3π/2) −√
1 − sin(π/2))
−2√
2(
√
1 − sin(2π) −√
1 − sin(3π/2))
= −2√
2 (0 − 1) + 2√
2(√
2 − 0)
− 2√
2(
1 −√
2)
= 2√
2 + 4 − 2√
2 + 4 = 8
11