section 3 arc length. definition let x = f(t), y = g(t) and that both dx/dt and dy/dt exist on [t...
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Section 3
Arc Length
Definition
Let x = f(t), y = g(t) and that both dx/dt and dy/dt exist on [t1,t2]
The arc length of the curve having the above parametric equations on the interval [t1,t2] is defined to
be:
dtt
tdtdy
dtdx
2
1
22 )()(
Example (1)
Find the arc length L of the curve:
x=rcost, y=rsint ; 0 ≤ t ≤ 2π, where r is a constant ( what does that represent?)
Solution:
?
2)cos()sin[(2
0
22
Why
rdttrtrL
Example (2)
Find the arc length L of the curve:
x=r(t-sint), y=r(1-cost) ; 0 ≤ t ≤ 2π, where r is a constant
Solution:
dx/dt = ?
dy/dt=?
rrrt
r
dtt
r
Whydtt
r
Whydtt
r
Whydttr
Whydttr
WhydttrtrL
8]1)1([4)0coscos[4)2
cos(22
2
1
2sin22
?**2
sin2
?*2
sin22
?cos12
?cos22
?)sin(])cos1([
20
2
0
2
0
2
0
2
2
0
2
0
2
0
222
20;2
sin2
sin
?20;02
sin
2020
:
?**2
sin22
sin22
cos12
sin2
2sin21
2sin)
2sin1(
2sin
2coscos:
?*2
sin22cos12
2
2
0
2
0
2
2
22222
2
0
22
0
ttt
Whytt
tt
Answer
Whydtt
rdtt
r
tt
ttttttAnswer
Whydtt
rdttr
DefinitionLet x = f(t), y = g(t) and that both dx/dt and dy/dt exist on [t1,t2]
Assume that g is non-negative and the derivatives of f and g are continuos on that interval.
The surface area resulting from the rotation the curve having the above parametric equations about the x-axis on the interval [t1,t2] is defined to be:
dtds
wheredsydtyS
dtdy
dtdx
t
t
t
tdtdy
dtdx
22
22
)()(
,,2)()(22
1
2
1
Example (1)
Find the surface area of the sphere with radius r !
Solution:
Consider the semicircle:
x = r cost , y = r sint ; 0 ≤ t ≤ π, where r is a
constant
The rotation of this semicircle results in a sphere
of radius r
220
2
0
2
0
22
22
4)11(2cos2
?sin2
?)cos()sin(sin2
)()(2
cos,sin
0;sin,cos
2
1
rrtr
Whydttr
Whydttrtrtr
dtyS
trdt
dytr
dt
dx
ttrytrx
t
tdtdy
dtdx