Probably for the next year

Calculus Level 1

Find the 2016-th derivative of sin 1 ( x ) \sin ^{ -1 }{ (x) } at x = 0 x=0 .

Give your answer to 3 decimal places.

Image Credit: Flickr Richard Stocker .


The answer is 0.000.

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3 solutions

Harry Ray
Jun 29, 2016

Directly calculating the derivatives of arcsine is difficult, so we'll solve this problem indirectly. First, note that the first derivative of arcsine is: d d x arcsin ( x ) = 1 1 x 2 \frac{\mathrm{d}}{\mathrm{d}x} \arcsin(x) = \frac{1}{\sqrt{1 - x^2}} We expand the right-hand side as a binomial series: 1 1 x 2 = 1 + ( 1 2 ) ( x 2 ) + ( 1 2 3 2 ) 1 2 ( x 2 ) 2 + ( 1 2 3 2 5 2 ) 1 2 3 ( x 2 ) 3 + = 1 + ( 1 2 ) x 2 + ( 1 3 2 4 ) x 4 + ( 1 3 5 2 4 6 ) x 6 + ( 1 3 5 7 2 4 6 8 ) x 8 + \begin{aligned} \frac{1}{\sqrt{1 - x^2}} &= 1 + \left(-\frac{1}{2}\right)\left(-x^2\right) + \left(-\frac{1}{2} \cdot -\frac{3}{2}\right)\frac{1}{2}\left(-x^2\right)^2 + \left(-\frac{1}{2} \cdot -\frac{3}{2} \cdot -\frac{5}{2}\right)\frac{1}{2 \cdot 3}\left(-x^2\right)^3 + \cdots \\ &= 1 + \left(\frac{1}{2}\right) x^2 + \left(\frac{1 \cdot 3}{2 \cdot 4}\right) x^4 + \left(\frac{1 \cdot 3 \cdot 5}{2 \cdot 4 \cdot 6}\right) x^6 + \left(\frac{1 \cdot 3 \cdot 5 \cdot 7}{2 \cdot 4 \cdot 6 \cdot 8}\right) x^8 + \cdots \end{aligned} This can now be integrated term-by-term to give a series for arcsine. arcsin ( x ) = x + ( 1 2 ) x 3 3 + ( 1 3 2 4 ) x 5 5 + ( 1 3 5 2 4 6 ) x 7 7 + ( 1 3 5 7 2 4 6 8 ) x 9 9 + \begin{aligned} \arcsin(x) &= x + \left(\frac{1}{2}\right) \frac{x^3}{3} + \left(\frac{1 \cdot 3}{2 \cdot 4}\right) \frac{x^5}{5} + \left(\frac{1 \cdot 3 \cdot 5}{2 \cdot 4 \cdot 6}\right) \frac{x^7}{7} + \left(\frac{1 \cdot 3 \cdot 5 \cdot 7}{2 \cdot 4 \cdot 6 \cdot 8}\right) \frac{x^9}{9} + \cdots \end{aligned} This, then, must be the Taylor (actually Maclaurin) series for arcsine. But, since this is the Taylor series, we have the following equality: n = 0 d n arcsin ( x ) d x n x = 0 x n n ! = x + ( 1 2 ) x 3 3 + ( 1 3 2 4 ) x 5 5 + ( 1 3 5 2 4 6 ) x 7 7 + ( 1 3 5 7 2 4 6 8 ) x 9 9 + \sum_{n=0}^\infty \left.\frac{\mathrm{d}^n\arcsin(x)}{\mathrm{d}x^n}\right|_{x=0} \frac{x^n}{n!} = x + \left(\frac{1}{2}\right) \frac{x^3}{3} + \left(\frac{1 \cdot 3}{2 \cdot 4}\right) \frac{x^5}{5} + \left(\frac{1 \cdot 3 \cdot 5}{2 \cdot 4 \cdot 6}\right) \frac{x^7}{7} + \left(\frac{1 \cdot 3 \cdot 5 \cdot 7}{2 \cdot 4 \cdot 6 \cdot 8}\right) \frac{x^9}{9} + \cdots The coefficients on both sides must be equal. In particular, we note that the coefficients of even powers of x x on the right-hand side are all zero. Equating the coefficients of x 2016 x^{2016} gives us the following: d 2016 arcsin ( x ) d x 2016 x = 0 = 2016 ! 0 = 0 \left.\frac{\mathrm{d}^{2016}\arcsin(x)}{\mathrm{d} x^{2016}}\right|_{x=0} = 2016! \cdot 0 = \boxed{0}

Samardeep Sarna
Nov 19, 2015

'Even'th derivatives are zero, 'odd'th derivatives are 1

Did it the same way

Prithwish Roy - 4 years, 3 months ago

If the nth derivative of the sine function is a multiple of 4, it is just equal to itself. In this case its easy to note that the 2016th derivative is a multiple of 4. Hence, 0 should be the answer. :)

Wrong. It is referring to the arcsine function, not sine function.

Pi Han Goh - 5 years, 9 months ago

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Is it ok if I used explicit differentiation?

Rindell Mabunga - 5 years, 9 months ago

The n t h nth derivative of a r c s i n x arcsinx at x = 0 x = 0 would be ( 1 ) n + 1 ( 2 n 3 ) ! 2 n 2 ( n 2 ) ! × 2 n 1 (-1)^{n+1}\frac{(2n-3)!}{2^{n-2}(n-2)!\times2^{n-1}} Thus for the n = 2016 t h n=2016th derivative we'd get ( 4029 ) ! 2 4029 × ( 2014 ) ! \frac{-(4029)!}{2^{4029}\times(2014)!}

Rob Matuschek - 5 years, 9 months ago

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Nope. You just need to show that arcsine function is an odd function then the even powered derivative of it is 0.

Pi Han Goh - 5 years, 9 months ago

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@Pi Han Goh Of course! Good call.

Rob Matuschek - 5 years, 9 months ago

@Pi Han Goh Exactly....

Vighnesh Raut - 5 years, 9 months ago

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