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Homework Help: Help with This Derivative

  1. Jun 16, 2012 #1
    Does somebody want to help me with differentiating this function:

    (0.003 + 0.004[itex]\sqrt{2}[/itex])∏r2 + 0.004∏r [(2250 - ∏r3) / 3∏r2)]

    Originally I tried simplifying it by distributing the 0.004∏r into the fraction, but whenever I did that the two expressions were never equivalent when I checked them on a graphing calculator. Then I tried using the product and quotient rules to differentiate the second term and the power rule for the first, but based on the graph that was not the correct derivative. I found that the derivative had only one zero at -1 which doesn't make sense because the original function clearly has a local minimum at around x = 4. Any help is appreciated, this is really bugging me. :s
  2. jcsd
  3. Jun 16, 2012 #2


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    What did you get for your function when you did the following?
  4. Jun 16, 2012 #3
    Are you sure there wasn't an algebra mistake? Be sure to double check your signs, a missed negative sign can be disasterous. Also double check your powers.

    Did you notice that after the plus sign, the pi and r cancel with the pi and one r in the denominator? Makes it so you only need to use the quotient rule in the second term.

    Before differentiation, it simplifies to: N*r^2 - M*r +L*(1/r) where N, M, and L are just numbers.
  5. Jun 16, 2012 #4
    After simplifying I get:
    (0.003 + 0.004√2) ∏r2 + 3/r - (4∏r2) / 300

    Which is equivalent now according to a graphing calculator.

    Differentiating, I get:
    (0.006 + 0.008√2) ∏r - 3r-2 - 1500/∏r3

    But thats not right. i.e. it has positive values when the slope of the function is negative. :/
  6. Jun 16, 2012 #5
    Sorry, I missed an r in my earlier calculation...
  7. Jun 16, 2012 #6
    Yeah I noticed. No worries. :tongue: Do you know what's wrong with the derivative. I found just with the power rule and the quotient rule on the last term.
  8. Jun 16, 2012 #7
    Where did you get that 4/300 from? Shouldn't it be 1/750?

    The 4/300 has a decimal expansion of 0.01333333... and the 1/750 is 0.0013333... I think you missed a zero. :)
  9. Jun 16, 2012 #8
    I typed that wrong, I did get 4/3000 (1/750).
  10. Jun 16, 2012 #9
    Ok...that makes sense. :) Where is that 1/r^3 coming from in your derivative?
  11. Jun 16, 2012 #10
    Using the quotient rule on the 4∏r2 / 3000 term. The denominator of the differentiated expression is 16∏2r4. A pi and an r cancel from the numerator which was 8∏r(3000).
  12. Jun 16, 2012 #11
    Quotient rule? You have the first two terms in the derivative right...the r and r^-2 terms...but the last one -1500/(pi*r^3) is wrong. That term should be your -8*pi*r/3000. You shouldn't need the quotient rule at all, just the power rule.
  13. Jun 16, 2012 #12


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    It looks like you forgot to multiply by 0.004 .
  14. Jun 16, 2012 #13
    No, I multiplied that in at the very beginning. I scaled up the fraction by 1000 times so I had a 4 in the numerator rather than a decimal. After fixing what Reptilian mentioned, my derivative looks correct, and the zero is at the x-value of that local minimum, so I think it's right.
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