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Math Help - Quick Chain Rule Question

  1. #1
    Junior Member cinder's Avatar
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    Quick Chain Rule Question

    I have a homework problem similar to this, but I was unsure about the example in the book:

    Find F'(z) if F(z) = (2z + 5)^3(3z - 1)^4

    The solution steps are as follows:

    F'(z) = (2z + 5)^3 \times 4(3z - 1)^3(3) + (3z - 1)^4 \times 3(2z + 5)^2(2)
    = 6(2z + 5)^2(3z - 1)^3[2(2z + 5) + (3z - 1)]
    = 6(2z + 5)^2(3z - 1)^3(7z + 9)

    I get a little lost during the second step, and where did that 6 come from?
    Last edited by cinder; February 20th 2006 at 01:20 PM.
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  2. #2
    Junior Member cinder's Avatar
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    I think I figured it out. It looks like they factored, which I didn't do, but it gives the same answer.
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  3. #3
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    F(z) = (2z + 5)^3(3z - 1)^4
    This uses the Product and Chain Rule.
    Notice the derivative of (2z+5)^3 is 6(2z+5)^2. Now you get the six because your bring down the exponent (which is three) and multiply by the derivative of the inside (which is two) thus you get 6.
    Similarly the derivative of (3z-1)^4 is 12(3z-1)^3.

    Now by the product rule,
    u'v+uv' and already found u',v' in the last paragraph. Thus,
    6(2z+5)^2(3z-1)^4+12(2z+5)^3(3z-1)^3
    Thus, factor,
    6(2z+5)^2(3z-1)^3((3z-1)+2(2z+5))
    Which becomes,
    6(2z+5)^2(3z-1)^3(3z-1+4z+10)
    Which becomes,
    6(2z+5)^2(3z-1)^3(7z+9)
    Q.E.D.
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