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Delphi27
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Homework Statement
Integrate y/(x^2+y^2) for x^2+y^2<1 and y> 1/2 ; use change of variables to polar coordinates
Homework Equations
THe above
The Attempt at a Solution
the variables transform as
y=rsinz
x=rcosz, where z is an angle between pi/6 and 5*pi/6 = which is the ArcSin of 1/2
That is sinz > 1/2 means pi/6 < z < 5*pi/6,
The Integral in polar coordinates becomes
Integral of [ r*sinz / { (r*sinz)^2 + (r*cosz)^2 } ] * r drdz
This simplifies to
Integral of [r*sinz / { r^2 * 1 } ]*r drdz, or
=Integral [ sinz drdz ]
Now y > 1/2 means that r must be between [ (1/2)^ + (cosz)^2 ] ^ (1/2) and 1
by the Pythagorean Theorum.
So i end up with the double integral
Integral rsinz dz evaluated at r = 1, minus same evaluated at
r = [ (1/2)^ + (cosz)^2 ] ^ (1/2)
So I then get Integral [ 1 - [ (1/2)^ + (cosz)^2 ] ^ (1/2) ] * sinz dz.
The book gives the answer (sqrt 3 ) - pi/3, that is after evaluating what I would think would be the above from z = pi/6 to 5*pi/6.
The integral I came up with after substituting the range of r stated as functions of z, does not seem to integrate and must be wrong -
Can anyone tell me where I went off track ?
Would be much appreciated - hours on this, and cannot get it.