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Math Help - Very very simple derivative

  1. #1
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    Very very simple derivative

    I m confused

    volume= (pie.r^3)/6

    dv/dr????


    Why dont we use product rule here???????????????
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  2. #2
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    Quote Originally Posted by santatrue View Post
    I m confused

    volume= (pie.r^3)/6

    dv/dr????


    Why dont we use product rule here???????????????
    You could if you want but since the volume is just a constant times r^3, and we know how to differentiate that directly (the power rule) we don't need to.

    That is after your first week of calculus you should know:

    \frac{d}{dx} kx^3= 3kx^2

    Using the product rule for this we need to know that \frac{dx}{dx}=1 , then it goes:

    \frac{d}{dx}x^3=\frac{d}{dx} x.x^2=1.x^2+x \frac{d}{dx}x^2=x^2+\frac{d}{dx}x.x=x^2+x[x.1+1.x]=3x^2


    RonL
    Last edited by CaptainBlack; June 22nd 2008 at 04:25 AM.
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