Find the 2016-th derivative of
sin
−
1
(
x
)
at
x
=
0
.
Give your answer to 3 decimal places.
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'Even'th derivatives are zero, 'odd'th derivatives are 1
Did it the same way
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.
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Is it ok if I used explicit differentiation?
The n t h derivative of a r c s i n x at x = 0 would be ( − 1 ) n + 1 2 n − 2 ( n − 2 ) ! × 2 n − 1 ( 2 n − 3 ) ! Thus for the n = 2 0 1 6 t h derivative we'd get 2 4 0 2 9 × ( 2 0 1 4 ) ! − ( 4 0 2 9 ) !
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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.
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@Pi Han Goh – Of course! Good call.
@Pi Han Goh – Exactly....
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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 x d arcsin ( x ) = 1 − x 2 1 We expand the right-hand side as a binomial series: 1 − x 2 1 = 1 + ( − 2 1 ) ( − x 2 ) + ( − 2 1 ⋅ − 2 3 ) 2 1 ( − x 2 ) 2 + ( − 2 1 ⋅ − 2 3 ⋅ − 2 5 ) 2 ⋅ 3 1 ( − x 2 ) 3 + ⋯ = 1 + ( 2 1 ) x 2 + ( 2 ⋅ 4 1 ⋅ 3 ) x 4 + ( 2 ⋅ 4 ⋅ 6 1 ⋅ 3 ⋅ 5 ) x 6 + ( 2 ⋅ 4 ⋅ 6 ⋅ 8 1 ⋅ 3 ⋅ 5 ⋅ 7 ) x 8 + ⋯ This can now be integrated term-by-term to give a series for arcsine. arcsin ( x ) = x + ( 2 1 ) 3 x 3 + ( 2 ⋅ 4 1 ⋅ 3 ) 5 x 5 + ( 2 ⋅ 4 ⋅ 6 1 ⋅ 3 ⋅ 5 ) 7 x 7 + ( 2 ⋅ 4 ⋅ 6 ⋅ 8 1 ⋅ 3 ⋅ 5 ⋅ 7 ) 9 x 9 + ⋯ 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 x n d n arcsin ( x ) ∣ ∣ ∣ ∣ x = 0 n ! x n = x + ( 2 1 ) 3 x 3 + ( 2 ⋅ 4 1 ⋅ 3 ) 5 x 5 + ( 2 ⋅ 4 ⋅ 6 1 ⋅ 3 ⋅ 5 ) 7 x 7 + ( 2 ⋅ 4 ⋅ 6 ⋅ 8 1 ⋅ 3 ⋅ 5 ⋅ 7 ) 9 x 9 + ⋯ The coefficients on both sides must be equal. In particular, we note that the coefficients of even powers of x on the right-hand side are all zero. Equating the coefficients of x 2 0 1 6 gives us the following: d x 2 0 1 6 d 2 0 1 6 arcsin ( x ) ∣ ∣ ∣ ∣ x = 0 = 2 0 1 6 ! ⋅ 0 = 0