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Math Help - implicit differentianion- the algebra of it

  1. #1
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    implicit differentianion- the algebra of it

    I hope I can get some help understanding the math of this problem. The problem is 2x^(2/3) + y^(2/3). I did this: d/dx (y^2/3) = -d/dx ((-2x^(2/3). I come up with dy/dx = -2x^(2/3)/y^(2/3). My graphing claculator says the answer is -2y^1/3)/x^(1/3). Can someone please help me understand the math that comes up with the solution on my calculator, or am I just doing this wrong?
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  2. #2
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    Quote Originally Posted by bosmith View Post
    I hope I can get some help understanding the math of this problem. The problem is 2x^(2/3) + y^(2/3). I did this: d/dx (y^2/3) = -d/dx ((-2x^(2/3). I come up with dy/dx = -2x^(2/3)/y^(2/3). My graphing claculator says the answer is -2y^1/3)/x^(1/3). Can someone please help me understand the math that comes up with the solution on my calculator, or am I just doing this wrong?

    The problem is ill-posed, I think: what is 2x^{2\slash3}+y^{2\slash3} equal to?

    Assuming the above is equal to a constant, then deriving implicitly:

    \frac{4}{3}x^{-1\slash3}dx+\frac{2}{3}y^{-1\slash3}dy=0\Longrightarrow \frac{dy}{dx}=-2\left(\frac{y}{x}\right)^{1\slash3}

    I'm guessing you took inadvertently \frac{dx}{dy} or perhaps you overlooked the minus signs in the powers.

    Tonio
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  3. #3
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    To find the derivative of x^n, we must subtract 1 from the exponent in addition to multiplying by n. This gives us

    \begin{aligned}<br />
2x^{2/3}+y^{2/3}&=0\\<br />
\frac{d}{dx}(2x^{2/3}+y^{2/3})&=0\\<br />
\frac{d}{dx}(2x^{2/3})+\frac{d}{dx}(y^{2/3})&=0\\<br />
2\cdot\frac{2}{3}x^{-1/3}+\frac{2}{3}y^{-1/3}\frac{dy}{dx}&=0\\<br />
\frac{dy}{dx}&=\left(-\frac{4}{3}x^{-1/3}\right)\left(\frac{2}{3}y^{-1/3}\right)^{-1}=-\frac{2x^{-1/3}}{y^{-1/3}}=-\frac{2y^{1/3}}{x^{1/3}}=-2\sqrt[3]{\frac{y}{x}}.<br />
\end{aligned}
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  4. #4
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    This is a grateful thanks to Tonio and Scott H

    Thank you both very much. I have been studying Calculus I, so much I sometimes forget the basics. The question came from a sample problem from my Instructor. There was no equal sign in the problem, somewhat puzzling. Doesn't matter though because your assistance put me on track. I simply forgot to subtract (-1) from the exponents. Once you showed me that, I now understand what I failed to do. Thanks.
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