Solve the given partial differential equation

chwala
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Homework Statement
Find the solutions satisfying ##2u_x-6u_y=0## given ##u(0,y)=\sin y##.
Relevant Equations
method of characteristics
Looking at pde today- your insight is welcome...

##η=-6x-2y##

therefore,

##u(x,y)=f(-6x-2y)##

applying the initial condition ##u(0,y)=\sin y##; we shall have

##\sin y = u(0,y)=f(-2y)##

##f(z)=\sin \left[\dfrac{-z}{2}\right]##

##u(x,y)=\sin \left[\dfrac{6x+2y}{2}\right]##
 
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Excellent. In short
u(x,y)=u(\eta(x,y))
u_x=\frac{du}{d\eta}\eta_x
u_y=\frac{du}{d\eta}\eta_y
From the condition given
\eta_x=3\eta_y
Thus we can make
\eta=3x+y
\eta(0,y)=y
From the boundary condition given
u=\sin\eta=\sin(3x+y)
 
just a quick question, can we also approach this using;

##u(ξ,η )=f(ξ)##

then it follows that,

##u(x,y)=f(2x-6y)##

##\sin y=u(0,y)=f(-6y)##

##f(z)=\sin\left[\dfrac{-z}{6}\right]##

##u(x,y)=\sin\left[\dfrac{-2x+6y}{6}\right]##
 
anuttarasammyak said:
Excellent. In short
u(x,y)=u(\eta(x,y))
u_x=\frac{du}{d\eta}\eta_x
u_y=\frac{du}{d\eta}\eta_y
From the condition given
\eta_x=3\eta_y
Thus we can make
\eta=3x+y
\eta(0,y)=y
From the boundary condition given
u=\sin\eta=\sin(3x+y)
Thus we can make
\eta=3x+y
\eta(0,y)=y
From the boundary condition given
u=\sin\eta=\sin(3x+y)

It is interesting on how you applied the boundary condition here... my understanding is that it applies to ##u(x,y)## looks like in your case it applies to ##η(x,y)##...the steps before this are quite clear.
 
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Please find attached the sketch for explanation. Best.
img20230109_22334057.jpg
 
chwala said:
just a quick question, can we also approach this using;

##u(ξ,η )=f(ξ)##

then it follows that,

##u(x,y)=f(2x-6y)##

##\sin y=u(0,y)=f(-6y)##

##f(z)=\sin\left[\dfrac{-z}{6}\right]##

##u(x,y)=\sin\left[\dfrac{-2x+6y}{6}\right]##
From this
u_x=-\frac{1}{3}\cos\{-\frac{x}{3}+y\}
u_y=\cos\{-\frac{x}{3}+y\}
So -3u_x=u_y
which is diffent from the problem statement.
 
@anuttarasammyak can you kindly show how you applied the boundary conditions? As requested in post ##4##...
 
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