Oh no! Fractional derivatives!

Calculus Level 3

f ( x ) = x 3 5 \large f(x)=\sqrt[5]{x^3}

Given that g ( x ) = f ( 0.5 ) ( x ) g(x) = f^{(0.5)}(x) , find g ( 1.5 ) ( 32 ) g^{(1.5)}(32)

Notation: f ( n ) ( x ) f^{(n)}(x) denotes the n th n^{\text{th}} derivative of f f at x x


The answer is -0.001875.

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1 solution

James Watson
Aug 18, 2020

We know that g ( x ) = d 0.5 d x 0.5 ( f ( x ) ) g(x) = \cfrac{d^{0.5}}{dx^{0.5}}\big( f(x) \big)

If we take the 1. 5 th 1.5^{\text{th}} derivative of g ( x ) g(x) , we get d 1.5 d x 1.5 ( d 0.5 d x 0.5 ( f ( x ) ) ) = d 1.5 + 0.5 d x 1.5 + 0.5 ( f ( x ) ) = d 2 d x 2 ( x 3 5 ) = 6 25 x 7 5 \begin{aligned} \frac{d^{1.5}}{dx^{1.5}} \left( \frac{d^{0.5}}{dx^{0.5}}\big( f(x) \big)\right) &= \frac{d^{1.5 + 0.5}}{dx^{1.5 + 0.5}}\big( f(x) \big) \\ &= \frac{d^{2}}{dx^{2}}\left( x^{\frac{3}{5}} \right) \\ &= -\frac{6}{25x^{\frac{7}{5}}} \end{aligned}

Now we can plug in 32 32 and the answer is 6 25 ( 32 ) 7 5 = 0.001875 -\frac{6}{25(32)^{\frac{7}{5}}} = \green{\boxed{-0.001875}}

Nice pun for the title!

Yajat Shamji - 9 months, 3 weeks ago

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