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**Tweety** thanks guys, I thought d = r^2, so thats why I was stuck.

I also need help on the second part, ;

from this derive an expression that gives the resolution of the density measurement $\displaystyle \Delta p $ in terms of the resolution of the mass and diameter measurements $\displaystyle \Delta m $ and $\displaystyle \Delta d $ respectively.

my booklet gives me this equation for resolution $\displaystyle \Delta p = | (\frac{\delta p}{\delta m})_{v} \Delta m | + | (\frac{\delta p }{\delta v }) _{m} \Delta V | $

I am not even sure what exactly is being asked here, but I did manage to work out $\displaystyle \frac{\delta p}{\delta m} $

density = $\displaystyle p = \frac{m}{v} $ and from the rearranged volume formula $\displaystyle v = \frac{1}{6} \pi d^{3} $

so if I jut plug in v into density formula, than differentiate I get $\displaystyle p = \frac{6m}{\pi d^{3}} $

$\displaystyle \frac{\delts p}{\delta m} = \frac{-18m}{\pi d^{4}} $

If this is correct, than can someone please help me with the other part of the equation, deltap/deltaaV

thank you