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anonymous

  • 5 years ago

suppose f(x)+ x^4[f(x)]^3=1028 and f(2)=4, find f`(2) How do I start this?

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  1. bahrom7893
    • 5 years ago
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    YAY differentiation!

  2. bahrom7893
    • 5 years ago
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    F(x) + x^4 [f(x)]^3 = 1028

  3. bahrom7893
    • 5 years ago
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    let me replace f(x) by y so that its easier to type

  4. bahrom7893
    • 5 years ago
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    y + x^4*y^3 = 1028; Use implicit differentiation: dy/dx + (x^4 * 3y^2 *dy/dx + 4x^3 * y^3) = 0

  5. bahrom7893
    • 5 years ago
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    I used the chain rule to differentiate that x^4*y^3

  6. bahrom7893
    • 5 years ago
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    So now just replace dy/dx by f'(x) and y by f(x), and then find the values at 2: f'(2) + (2^4 * 3 * [f(2)]^2 * f'(2) + 4*2^3 * [f(2)]^3) = 0

  7. bahrom7893
    • 5 years ago
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    Replace f(2) by 4, because f(2) = 4 and simplify a bit: f ' (2) + (16 * 3 * 4^2 * f ' (2) + 4 * 8 * 4^3) = 0 f ' ( 2 ) + 768 * f ' ( 2 ) + 2048 = 0

  8. bahrom7893
    • 5 years ago
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    wait sorry i made a typo

  9. bahrom7893
    • 5 years ago
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    checkin my arithmetic... i hate typing out math solutions

  10. bahrom7893
    • 5 years ago
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    okay so here we go: \[y + x^4 * y^3 = 1028\] \[dy/dx + x^4 * 3y^2 (dy/dx) + y^3 *4x^3 = 0\] \[dy/dx( 1 + x^4 * 3y^2 ) = -y^3 * 4x^3\]

  11. bahrom7893
    • 5 years ago
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    \[dy/dx = (-y^3*4x^3)/(1+x^4*3y^2)\]

  12. bahrom7893
    • 5 years ago
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    okay now you know that f(2) is 4, therefore to find f' or dy/dx at x = 2, replace all ys by 4 and xs by 2

  13. bahrom7893
    • 5 years ago
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    \[dy/dx = (-4^3 * 4*2^3) / (1 + 2^4 * 3* 4^2)\]

  14. bahrom7893
    • 5 years ago
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    dy/dx = (-2048)/769

  15. bahrom7893
    • 5 years ago
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    dy/dx = -2048/769

  16. bahrom7893
    • 5 years ago
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    I don't like the number.. don't know why, but i triple checked my math...

  17. bahrom7893
    • 5 years ago
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    WOOOT! Thanks for fanning, whoever you are, lol I'm a superhero now!

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