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Math Help - help with an integral

  1. #1
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    help with an integral

    How do I find dF(x)/dx where
    F(x) = integral sin(t^2) dt
    [from 2^x to 3+x^3]

    I'm not sure what dF(x) is (like the notation).
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  2. #2
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    Re: help with an integral

    Quote Originally Posted by Sneaky View Post
    How do I find dF(x)/dx where
    F(x) = integral sin(t^2) dt
    [from 2^x to 3+x^3]

    I'm not sure what dF(x) is (like the notation).
    \frac{d}{{dx}}\int_{g(x)}^{h(x)} {f(t)dt}  = f \circ h(x)h'(x) - f \circ g(x)g'(x)
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  3. #3
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    Re: help with an integral

    So dF(x)/dx = f o (3+x^3)(3x^2) - f o (2^x)(2^x)(ln2)
    = f o (9x^2 + 3x^5) - f o ((2^(2x))(ln2))
    = sin(9x^2 + 3x^5)^2 - sin((2^(2x))(ln2))^2
    Is this right? Can it be more simplified?
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  4. #4
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    Re: help with an integral

    Quote Originally Posted by Sneaky View Post
    So dF(x)/dx = f o (3+x^3)(3x^2) - f o (2^x)(2^x)(ln2)
    = f o (9x^2 + 3x^5) - f o ((2^(2x))(ln2))
    = sin(9x^2 + 3x^5)^2 - sin((2^(2x))(ln2))^2
    Is this right? Can it be more simplified?
    The derivative is \left( {3x^2 } \right)\sin \left( {\left[ {3 + x^2 } \right]^2 } \right) - 2^x \ln (2)\sin \left( {\left[ {3 + x^2 } \right]^2 } \right)
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  5. #5
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    Re: help with an integral

    Now i'm a bit lost,
    Is this true
    dF(x)/dx = f o [(3+x^3)(3x^2)] - f o [(2^x)(2^x)(ln2)]
    or
    dF(x)/dx = f o (3+x^3)[(3x^2)] - f o (2^x)[(2^x)(ln2)]
    ?
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  6. #6
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    Re: help with an integral

    Quote Originally Posted by Sneaky View Post
    Now i'm a bit lost,
    Is this true
    dF(x)/dx = f o [(3+x^3)(3x^2)] - f o [(2^x)(2^x)(ln2)]
    or
    dF(x)/dx = f o (3+x^3)[(3x^2)] - f o (2^x)[(2^x)(ln2)]
    ?
    Please seek a live session with your instructor.
    There ask your questions.
    Your misunderstanding puts you beyond our ability to help you
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  7. #7
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    Re: help with an integral

    Just in case a picture helps...

    Quote Originally Posted by Sneaky View Post
    I'm not sure what dF(x) is (like the notation).
    dF(x)/dx means the derivative, with respect to x, of the function F(x), i.e. d/dx of F(x).

    You were also thrown by the composition notation in Plato's formula. Below, I shall bother you with another notation for composition of functions, which you won't necessarily find any easier. However, do note that Plato's formula relies on the fact that...

    \int_{x}^{y}\ f(t)\ dt\ =\ \int_a^{y}\ f(t)\ dt\ -\ \int_a^{x}\ f(t)\ dt

    ... for some constant a. So you may as well split your problem into two...

    \int_{2^x}^{3+x^3}\ sin(t^2)\ dt\ =\ \int_a^{3+x^3}\ sin(t^2)\ dt\ -\ \int_a^{2^x}\ sin(t^2)\ dt

    Now you apply Fundamental theorem of calculus - Wikipedia, the free encyclopedia, which kind of says...



    ... if the straight line means differentiating (downwards) with respect to x (the continuous line) or with respect to the dashed balloon (the dashed line). The really confusing bit here, with or without the diagram, is the 'dummy variable', t. I googled a helpful explanation of this, can't find it right now. But the left diagram does basically tell it like it is, i.e. when differentiating the integral with respect to the upper limit (x) you get just the integrand said about x instead of about t or whatever.

    The dashed is because we might need...



    ... the chain rule, as we do here. Writing...

    \int_a^{3+x^3}\ sin(t^2)\ dt

    ... as...



    ... and then working downwards through the chain rule, we get...



    And you might like to see if you can do similarly for the second part - i.e. the blanks here...




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