Components using partial derivatives

[transgalactic]

here is the question:
http://i44.tinypic.com/xe53tc.gif

here is the solution:
http://i43.tinypic.com/2nuokfq.gif
my first question regarding this whole thing is.
why when the doing the partial derivative by "r" we don't multiply by minus
the formula says (minus derivative)
but all they do is making a derivative without multiplying by minus?

 

[tiny-tim]

Hi transgalactic!

here is the question:
http://i43.tinypic.com/2nuokfq.jpg

here is the solution:
http://i43.tinypic.com/2nuokfq.gif

erm … they're the same …

get some sleep! :zzz:​

why when the doing the partial derivative by "r" we don't multiply by minus
the formula says (minus derivative)
but all they do is making a derivative without multiplying by minus?

?? but they have multiplied by the minus …

∂/∂r (1/r²) = -2/r³, and ∂/∂θ (cosθ) = -sinθ

 

[transgalactic]

but we are doing a derivative by 'r'(partial derivative)
we don't touch \theta ,its constant

 

[transgalactic]

i changed the original post
now its ok

 

[tiny-tim]

but we are doing a derivative by 'r'(partial derivative)
we don't touch \theta ,its constant

ok, but still ∂/∂r (1/r²) = -2/r³

 

[transgalactic]

thanks