Showing a vector field is imcompressible

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Homework Statement


attachment.php?attachmentid=455997&d=1440616258.jpg


Homework Equations

The Attempt at a Solution



As you can see, the solution is shown just below the question.

Essentially, I don't understand how the x, y and z component of the vector field has been separated because the numerator of the vector field's fraction is: (x^2 + y^2 + z^2)^(1/2)

It seems like they've just taken x^2 out and square rooted it to get 'x', but you can't do that, can you?
 
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question dude said:

Homework Statement


attachment.php?attachmentid=455997&d=1440616258.jpg


Homework Equations

The Attempt at a Solution



As you can see, the solution is shown just below the question.

Essentially, I don't understand how the x, y and z component of the vector field has been separated because the numerator of the vector field's fraction is: (x^2 + y^2 + z^2)^(1/2)

It seems like they've just taken x^2 out and square rooted it to get 'x', but you can't do that, can you?

That isn't what they have done. The numerator is bold faced ##\bf{r}## and the denominator is ##r##. The first is the vector and the second its magnitude. r = xi + yj + zk.
 
LCKurtz said:
That isn't what they have done. The numerator is bold faced ##\bf{r}## and the denominator is ##r##. The first is the vector and the second its magnitude. r = xi + yj + zk.

Do I treat the non-bold r as a constant?

If sub in bold r, I get:

G = [(x^2 + y^2 + z^2)^0.5 ] / 4*pi*r^3
 
question dude said:
Do I treat the non-bold r as a constant?

If sub in bold r, I get:

G = [(x^2 + y^2 + z^2)^0.5 ] / 4*pi*r^3

I just noticed in your graphic under (b) they have ##{\bf r} = \sqrt{x^2+y^2+z^2}## That should not have been a bold face r. The bold face r represents the vector and the plain r its magnitude, which is not constant.
 
Prove $$\int\limits_0^{\sqrt2/4}\frac{1}{\sqrt{x-x^2}}\arcsin\sqrt{\frac{(x-1)\left(x-1+x\sqrt{9-16x}\right)}{1-2x}} \, \mathrm dx = \frac{\pi^2}{8}.$$ Let $$I = \int\limits_0^{\sqrt 2 / 4}\frac{1}{\sqrt{x-x^2}}\arcsin\sqrt{\frac{(x-1)\left(x-1+x\sqrt{9-16x}\right)}{1-2x}} \, \mathrm dx. \tag{1}$$ The representation integral of ##\arcsin## is $$\arcsin u = \int\limits_{0}^{1} \frac{\mathrm dt}{\sqrt{1-t^2}}, \qquad 0 \leqslant u \leqslant 1.$$ Plugging identity above into ##(1)## with ##u...

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