May be too lengthy 2

Calculus Level 5

0 1 arctan ( x ) ( ( ln x ) 3 + 3 ( ln x ) 2 ) d x = A B π C \large{\displaystyle \int ^{1}_{0} \arctan(x) \left( (\ln x)^3+3 (\ln x)^2 \right)dx= \frac{A}{B} \pi^{C}}

where A , B , C A,B,C are positive integers and A , B A,B being co-prime.

Find A + B + C A+B+C


The answer is 1931.

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1 solution

Mark Hennings
Feb 3, 2016

Using the Maclaurin series expansion for tan 1 x \tan^{-1}x we define f ( a ) = 0 1 x a tan 1 x d x = n = 0 ( 1 ) n ( 2 n + 1 ) ( 2 n + a + 2 ) , a 1 , f(a) \; = \; \int_0^1 x^a \tan^{-1}x\,dx \; = \; \sum_{n=0}^\infty \frac{(-1)^n}{(2n+1)(2n+a+2)} \;, \qquad a \ge -1\;, so that f ( j ) ( a ) = ( 1 ) j j ! n = 0 ( 1 ) n ( 2 n + 1 ) ( 2 n + a + 2 ) j + 1 , j 0 , f^{(j)}(a) \; = \; (-1)^j j! \sum_{n=0}^\infty \frac{(-1)^n}{(2n+1)(2n+a+2)^{j+1}} \;, \qquad j \ge 0 \;, (all these series converge absolutely and uniformly for a 1 a \ge -1 , so the term-by-term differentiation is justified). Thus f ( 0 ) = 2 n = 0 ( 1 ) n ( 2 n + 1 ) ( 2 n + 2 ) 3 f ( 0 ) = 6 n = 0 ( 1 ) n ( 2 n + 1 ) ( 2 n + 2 ) 4 f''(0) \; = \; 2\sum_{n=0}^\infty \frac{(-1)^n}{(2n+1)(2n+2)^3} \qquad f'''(0) \; =\; -6\sum_{n=0}^\infty \frac{(-1)^n}{(2n+1)(2n+2)^4} and so 0 1 tan 1 x [ ( ln x ) 3 + 3 ( ln x ) 2 ] d x = f ( 0 ) + 3 f ( 0 ) = 6 n = 0 [ ( 1 ) n ( 2 n + 1 ) ( 2 n + 2 ) 3 ( 1 ) n ( 2 n + 1 ) ( 2 n + 2 ) 4 ] = 6 n = 0 ( 1 ) n ( 2 n + 1 ) ( 2 n + 2 ) 4 [ ( 2 n + 2 ) 1 ] = 6 n = 0 ( 1 ) n ( 2 n + 2 ) 4 = 3 8 n = 0 ( 1 ) n ( n + 1 ) 4 = 3 8 [ n = 1 1 n 4 2 n = 1 1 ( 2 n ) 4 ] = 3 8 [ ζ ( 4 ) 1 8 ζ ( 4 ) ] = 21 64 ζ ( 4 ) = 7 1920 π 4 , \begin{array}{rcl} \displaystyle \int_0^1 \tan^{-1}x\big[(\ln x)^3 + 3(\ln x)^2\big]\,dx & = & f'''(0) + 3f''(0) \\ & = & \displaystyle 6\sum_{n=0}^\infty \left[ \frac{(-1)^n}{(2n+1)(2n+2)^3} - \frac{(-1)^n}{(2n+1)(2n+2)^4}\right] \\ & = & \displaystyle 6\sum_{n=0}^\infty \frac{(-1)^n}{(2n+1)(2n+2)^4}\big[(2n+2)-1\big] \\ & = & \displaystyle 6\sum_{n=0}^\infty \frac{(-1)^n}{(2n+2)^4} \; = \; \frac38 \sum_{n=0}^\infty \frac{(-1)^n}{(n+1)^4} \\ & = & \displaystyle \frac38\left[ \sum_{n=1}^\infty \frac{1}{n^4} - 2\sum_{n=1}^\infty \frac{1}{(2n)^4}\right] \\ & = &\displaystyle \tfrac38\left[\zeta(4) - \tfrac18\zeta(4)\right] \; = \; \tfrac{21}{64}\zeta(4) \\ & = & \frac{7}{1920}\pi^4 \;, \end{array} making the answer 7 + 1920 + 4 = 1931 7 + 1920 + 4 \,=\, \boxed{1931} .

Be-lated Happy b'day Sir!

Harsh Shrivastava - 5 years, 4 months ago

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