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Line integral (flux)

  • Thread starter jahlin
  • Start date
  • #1
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Homework Statement


for [tex]\varphi[/tex](x,y)=2x+y+10 ,calculate the flux line integral...on a straight line from A(1,4) to B(5,1).



Homework Equations





The Attempt at a Solution


I tried to solve it but didnt get the right answer.
first i found the quation of the line which i found to be y=-(3/4)*x
and then i set x=t (parameter) and insert t into the y equation=-(3/4)*t. 1<t<5
dx=dt;dy=-(3/4)dt

and i used the formula for line integral which is [tex]\int[/tex](2x+y+10)*(dy,-dx)
i subtituted the values for x,y,dx and dt in to the above equation and integrated over 5 to 1..
the right answer is 92. i got -9.
 

Answers and Replies

  • #2
Borek
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By mistake I have removed answer by hgfalling, sorry about that. Here it comes:

The equation of the line is definitely not y=-3x/4: the point (1,4) is on the line!

I'm not sure what your formula for the line integral is at all; is that supposed to be a dot product?
I suggest you parametrize your line correctly as a function r(t); it seems you are on the right track with this. Then

[tex]
\intop_S f ds = \intop_a^b f(r(t)) |rsingle-quote(t)| dt
[/tex]
 

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