tough question in Line integral / Multivariable Calculas
Let $C$ be a curve in the $(x − y)$-plane. For every point $(x, y)$ of $C$
let $u(x, y)$ denote the unit vector in the direction of the tangent line
to $C$ at $(x, y)$. Let $S$ be the surface obtained by taking the union of
all straight line segments connecting $(1, 2, 3)$ to points of $C$. Express
the area of $S$ as an integral of the first type, on the curve $C$, of some
function of $x$ and $y$. (hint: try to use the function $u(x, y)$.)
really Hard question , i couldn't understand how to use the fact that line integral will help here since i don't have a function $f(x,y)$ to calculate $ int_{C} f(x(t),y(t))sqrt{x'(t)^2 + y'(t)^2 } dt$ also i can parameterize $S$ like that :
$S=:k(1-x(t),2-y(t),3) , kin[0,1]$ where $(x(t),y(t),0)$ is the curve $C$
and why $u(x,y)$ is given here.
Unit vector
integration multivariable-calculus
add a comment |
Let $C$ be a curve in the $(x − y)$-plane. For every point $(x, y)$ of $C$
let $u(x, y)$ denote the unit vector in the direction of the tangent line
to $C$ at $(x, y)$. Let $S$ be the surface obtained by taking the union of
all straight line segments connecting $(1, 2, 3)$ to points of $C$. Express
the area of $S$ as an integral of the first type, on the curve $C$, of some
function of $x$ and $y$. (hint: try to use the function $u(x, y)$.)
really Hard question , i couldn't understand how to use the fact that line integral will help here since i don't have a function $f(x,y)$ to calculate $ int_{C} f(x(t),y(t))sqrt{x'(t)^2 + y'(t)^2 } dt$ also i can parameterize $S$ like that :
$S=:k(1-x(t),2-y(t),3) , kin[0,1]$ where $(x(t),y(t),0)$ is the curve $C$
and why $u(x,y)$ is given here.
Unit vector
integration multivariable-calculus
add a comment |
Let $C$ be a curve in the $(x − y)$-plane. For every point $(x, y)$ of $C$
let $u(x, y)$ denote the unit vector in the direction of the tangent line
to $C$ at $(x, y)$. Let $S$ be the surface obtained by taking the union of
all straight line segments connecting $(1, 2, 3)$ to points of $C$. Express
the area of $S$ as an integral of the first type, on the curve $C$, of some
function of $x$ and $y$. (hint: try to use the function $u(x, y)$.)
really Hard question , i couldn't understand how to use the fact that line integral will help here since i don't have a function $f(x,y)$ to calculate $ int_{C} f(x(t),y(t))sqrt{x'(t)^2 + y'(t)^2 } dt$ also i can parameterize $S$ like that :
$S=:k(1-x(t),2-y(t),3) , kin[0,1]$ where $(x(t),y(t),0)$ is the curve $C$
and why $u(x,y)$ is given here.
Unit vector
integration multivariable-calculus
Let $C$ be a curve in the $(x − y)$-plane. For every point $(x, y)$ of $C$
let $u(x, y)$ denote the unit vector in the direction of the tangent line
to $C$ at $(x, y)$. Let $S$ be the surface obtained by taking the union of
all straight line segments connecting $(1, 2, 3)$ to points of $C$. Express
the area of $S$ as an integral of the first type, on the curve $C$, of some
function of $x$ and $y$. (hint: try to use the function $u(x, y)$.)
really Hard question , i couldn't understand how to use the fact that line integral will help here since i don't have a function $f(x,y)$ to calculate $ int_{C} f(x(t),y(t))sqrt{x'(t)^2 + y'(t)^2 } dt$ also i can parameterize $S$ like that :
$S=:k(1-x(t),2-y(t),3) , kin[0,1]$ where $(x(t),y(t),0)$ is the curve $C$
and why $u(x,y)$ is given here.
Unit vector
integration multivariable-calculus
integration multivariable-calculus
edited Jan 5 at 7:04
Mather
asked Jan 4 at 21:21
Mather Mather
1777
1777
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2 Answers
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$c(t) = (x(t),y(t), 0)\
frac {dc}{dt} = u(t)$
The parmeterization of $S$ should be
$S = (k +(1-k) x,2 k + (1-k) y, 3k)$
or
$((1-k) +k x,2(1- k) + k) y(t), 3(1-k))$
with
$0le k le 1$
$dS = |frac {partial S}{dk} times frac {partial S}{dt}|$
$frac {partial S}{dk} = (1-x, 2-y,3) = (1,2,3) - c(t)$
$frac {partial S}{dt} = (1-k)u(t)$
$int_0^1 (1-k) dk int_0^t |(1,2,3)times u(t) - c(t)times u(t)| dt$
$frac 12 int_0^t |(1,2,3)times u(t) - c(t)times u(t)| dt$
Thank you this is what I tried to do , I didn’t understand one thing , $ u(x(t),y(t)) $ is a unit vector shouldn’t we multiply this Vector function with the length of the tangent line to the curve to get the surface area ? or it doesn’t matter and if it doesn’t why
– Mather
Jan 5 at 7:00
add a comment |
This is a very non-rigorous derivation.
Consider a point $C(t) = (x(t),y(t),0) in C$ and a second point infinitesimally close to it:
$$C(t+dt) = (x(t+dt),y(t+dt),0) = (x(t)+dot{x}(t),dt,y(t)+dot{y}(t),dt,0)$$
We need to calculate the infinitesimal area $dA$ of the triangle with vertices $C(t),C(t+dt)$ and $(1,2,3)$.
The area is given by
begin{align}
dA &= frac12 |(C(t+dt) - C(t)) times ((1,2,3) - C(t))| \
&= frac12 |(dot{x},dt,dot{y},dt,0) times (1-x,2-y,3)|\
&= frac{dt}2 |(dot{x},dot{y},0) times (1-x(t),2-y(t),3)|\
&= frac{dt}2 |(3dot{y},-3dot{x},(2-y)dot{x}+(x-1)dot{y})|\
end{align}
so $$A = int_C,dA = frac12int_{t}|(3dot{y}(t),-3dot{x}(t),(2-y(t))dot{x}(t)+(x(t)-1)dot{y}(t))|,dt$$
add a comment |
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2 Answers
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2 Answers
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$c(t) = (x(t),y(t), 0)\
frac {dc}{dt} = u(t)$
The parmeterization of $S$ should be
$S = (k +(1-k) x,2 k + (1-k) y, 3k)$
or
$((1-k) +k x,2(1- k) + k) y(t), 3(1-k))$
with
$0le k le 1$
$dS = |frac {partial S}{dk} times frac {partial S}{dt}|$
$frac {partial S}{dk} = (1-x, 2-y,3) = (1,2,3) - c(t)$
$frac {partial S}{dt} = (1-k)u(t)$
$int_0^1 (1-k) dk int_0^t |(1,2,3)times u(t) - c(t)times u(t)| dt$
$frac 12 int_0^t |(1,2,3)times u(t) - c(t)times u(t)| dt$
Thank you this is what I tried to do , I didn’t understand one thing , $ u(x(t),y(t)) $ is a unit vector shouldn’t we multiply this Vector function with the length of the tangent line to the curve to get the surface area ? or it doesn’t matter and if it doesn’t why
– Mather
Jan 5 at 7:00
add a comment |
$c(t) = (x(t),y(t), 0)\
frac {dc}{dt} = u(t)$
The parmeterization of $S$ should be
$S = (k +(1-k) x,2 k + (1-k) y, 3k)$
or
$((1-k) +k x,2(1- k) + k) y(t), 3(1-k))$
with
$0le k le 1$
$dS = |frac {partial S}{dk} times frac {partial S}{dt}|$
$frac {partial S}{dk} = (1-x, 2-y,3) = (1,2,3) - c(t)$
$frac {partial S}{dt} = (1-k)u(t)$
$int_0^1 (1-k) dk int_0^t |(1,2,3)times u(t) - c(t)times u(t)| dt$
$frac 12 int_0^t |(1,2,3)times u(t) - c(t)times u(t)| dt$
Thank you this is what I tried to do , I didn’t understand one thing , $ u(x(t),y(t)) $ is a unit vector shouldn’t we multiply this Vector function with the length of the tangent line to the curve to get the surface area ? or it doesn’t matter and if it doesn’t why
– Mather
Jan 5 at 7:00
add a comment |
$c(t) = (x(t),y(t), 0)\
frac {dc}{dt} = u(t)$
The parmeterization of $S$ should be
$S = (k +(1-k) x,2 k + (1-k) y, 3k)$
or
$((1-k) +k x,2(1- k) + k) y(t), 3(1-k))$
with
$0le k le 1$
$dS = |frac {partial S}{dk} times frac {partial S}{dt}|$
$frac {partial S}{dk} = (1-x, 2-y,3) = (1,2,3) - c(t)$
$frac {partial S}{dt} = (1-k)u(t)$
$int_0^1 (1-k) dk int_0^t |(1,2,3)times u(t) - c(t)times u(t)| dt$
$frac 12 int_0^t |(1,2,3)times u(t) - c(t)times u(t)| dt$
$c(t) = (x(t),y(t), 0)\
frac {dc}{dt} = u(t)$
The parmeterization of $S$ should be
$S = (k +(1-k) x,2 k + (1-k) y, 3k)$
or
$((1-k) +k x,2(1- k) + k) y(t), 3(1-k))$
with
$0le k le 1$
$dS = |frac {partial S}{dk} times frac {partial S}{dt}|$
$frac {partial S}{dk} = (1-x, 2-y,3) = (1,2,3) - c(t)$
$frac {partial S}{dt} = (1-k)u(t)$
$int_0^1 (1-k) dk int_0^t |(1,2,3)times u(t) - c(t)times u(t)| dt$
$frac 12 int_0^t |(1,2,3)times u(t) - c(t)times u(t)| dt$
answered Jan 4 at 21:59
Doug MDoug M
44.2k31854
44.2k31854
Thank you this is what I tried to do , I didn’t understand one thing , $ u(x(t),y(t)) $ is a unit vector shouldn’t we multiply this Vector function with the length of the tangent line to the curve to get the surface area ? or it doesn’t matter and if it doesn’t why
– Mather
Jan 5 at 7:00
add a comment |
Thank you this is what I tried to do , I didn’t understand one thing , $ u(x(t),y(t)) $ is a unit vector shouldn’t we multiply this Vector function with the length of the tangent line to the curve to get the surface area ? or it doesn’t matter and if it doesn’t why
– Mather
Jan 5 at 7:00
Thank you this is what I tried to do , I didn’t understand one thing , $ u(x(t),y(t)) $ is a unit vector shouldn’t we multiply this Vector function with the length of the tangent line to the curve to get the surface area ? or it doesn’t matter and if it doesn’t why
– Mather
Jan 5 at 7:00
Thank you this is what I tried to do , I didn’t understand one thing , $ u(x(t),y(t)) $ is a unit vector shouldn’t we multiply this Vector function with the length of the tangent line to the curve to get the surface area ? or it doesn’t matter and if it doesn’t why
– Mather
Jan 5 at 7:00
add a comment |
This is a very non-rigorous derivation.
Consider a point $C(t) = (x(t),y(t),0) in C$ and a second point infinitesimally close to it:
$$C(t+dt) = (x(t+dt),y(t+dt),0) = (x(t)+dot{x}(t),dt,y(t)+dot{y}(t),dt,0)$$
We need to calculate the infinitesimal area $dA$ of the triangle with vertices $C(t),C(t+dt)$ and $(1,2,3)$.
The area is given by
begin{align}
dA &= frac12 |(C(t+dt) - C(t)) times ((1,2,3) - C(t))| \
&= frac12 |(dot{x},dt,dot{y},dt,0) times (1-x,2-y,3)|\
&= frac{dt}2 |(dot{x},dot{y},0) times (1-x(t),2-y(t),3)|\
&= frac{dt}2 |(3dot{y},-3dot{x},(2-y)dot{x}+(x-1)dot{y})|\
end{align}
so $$A = int_C,dA = frac12int_{t}|(3dot{y}(t),-3dot{x}(t),(2-y(t))dot{x}(t)+(x(t)-1)dot{y}(t))|,dt$$
add a comment |
This is a very non-rigorous derivation.
Consider a point $C(t) = (x(t),y(t),0) in C$ and a second point infinitesimally close to it:
$$C(t+dt) = (x(t+dt),y(t+dt),0) = (x(t)+dot{x}(t),dt,y(t)+dot{y}(t),dt,0)$$
We need to calculate the infinitesimal area $dA$ of the triangle with vertices $C(t),C(t+dt)$ and $(1,2,3)$.
The area is given by
begin{align}
dA &= frac12 |(C(t+dt) - C(t)) times ((1,2,3) - C(t))| \
&= frac12 |(dot{x},dt,dot{y},dt,0) times (1-x,2-y,3)|\
&= frac{dt}2 |(dot{x},dot{y},0) times (1-x(t),2-y(t),3)|\
&= frac{dt}2 |(3dot{y},-3dot{x},(2-y)dot{x}+(x-1)dot{y})|\
end{align}
so $$A = int_C,dA = frac12int_{t}|(3dot{y}(t),-3dot{x}(t),(2-y(t))dot{x}(t)+(x(t)-1)dot{y}(t))|,dt$$
add a comment |
This is a very non-rigorous derivation.
Consider a point $C(t) = (x(t),y(t),0) in C$ and a second point infinitesimally close to it:
$$C(t+dt) = (x(t+dt),y(t+dt),0) = (x(t)+dot{x}(t),dt,y(t)+dot{y}(t),dt,0)$$
We need to calculate the infinitesimal area $dA$ of the triangle with vertices $C(t),C(t+dt)$ and $(1,2,3)$.
The area is given by
begin{align}
dA &= frac12 |(C(t+dt) - C(t)) times ((1,2,3) - C(t))| \
&= frac12 |(dot{x},dt,dot{y},dt,0) times (1-x,2-y,3)|\
&= frac{dt}2 |(dot{x},dot{y},0) times (1-x(t),2-y(t),3)|\
&= frac{dt}2 |(3dot{y},-3dot{x},(2-y)dot{x}+(x-1)dot{y})|\
end{align}
so $$A = int_C,dA = frac12int_{t}|(3dot{y}(t),-3dot{x}(t),(2-y(t))dot{x}(t)+(x(t)-1)dot{y}(t))|,dt$$
This is a very non-rigorous derivation.
Consider a point $C(t) = (x(t),y(t),0) in C$ and a second point infinitesimally close to it:
$$C(t+dt) = (x(t+dt),y(t+dt),0) = (x(t)+dot{x}(t),dt,y(t)+dot{y}(t),dt,0)$$
We need to calculate the infinitesimal area $dA$ of the triangle with vertices $C(t),C(t+dt)$ and $(1,2,3)$.
The area is given by
begin{align}
dA &= frac12 |(C(t+dt) - C(t)) times ((1,2,3) - C(t))| \
&= frac12 |(dot{x},dt,dot{y},dt,0) times (1-x,2-y,3)|\
&= frac{dt}2 |(dot{x},dot{y},0) times (1-x(t),2-y(t),3)|\
&= frac{dt}2 |(3dot{y},-3dot{x},(2-y)dot{x}+(x-1)dot{y})|\
end{align}
so $$A = int_C,dA = frac12int_{t}|(3dot{y}(t),-3dot{x}(t),(2-y(t))dot{x}(t)+(x(t)-1)dot{y}(t))|,dt$$
answered Jan 4 at 22:15
mechanodroidmechanodroid
27k62446
27k62446
add a comment |
add a comment |
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