Integration Problem

Calculus Level 4

100 × 1 9 11 + 9 8 2 ! 1 21 9 8 7 3 ! 1 31 + m = 4 9 ( 1 ) m ( 9 m ) 10 m + 1 1 10 11 + 10 9 2 ! 1 21 10 9 8 3 ! 1 31 + n = 4 10 ( 1 ) n ( 10 n ) 10 n + 1 = ? \large 100 \times \dfrac{1-\frac{9}{11}+\frac{9\cdot8}{2!}\frac{1}{21}-\frac{9\cdot8\cdot7}{3!}\frac{1}{31}+\sum_{m=4}^9(-1)^m\frac{\binom{9}{m}}{10m+1}}{1-\frac{10}{11}+\frac{10\cdot9}{2!}\frac{1}{21}-\frac{10\cdot9\cdot8}{3!}\frac{1}{31}+\sum_{n=4}^{10}(-1)^n\frac{\binom{10}{n}}{10n+1}} = \, ? Clarification : The numerator and the denominator contain the general term of summation after explicit 4 terms.

Notation : ! ! denotes the factorial notation. For example, 8 ! = 1 × 2 × 3 × × 8 8! = 1\times2\times3\times\cdots\times8 .


The answer is 101.

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1 solution

Rajen Kapur
Aug 17, 2016

Use 0 1 ( 1 x 10 ) p d x = \int_0^1(1-x^{10})^p \cdot dx = 1 p 11 + p ( p 1 ) 2 ! 1 21 p ( p 1 ) ( p 2 ) 3 ! 1 31 + p ( p 1 ) ( p 2 ) ( p 3 ) 4 ! 1 41 1 - \frac {p}{11}+\frac{p(p-1)}{2!}\frac{1}{21}-\frac{p(p-1)(p-2)}{3!}\frac {1}{31}+\frac {p(p-1)(p-2)(p-3)}{4!}\frac {1}{41}- . . . and so on as the value obtained at x=1 of the term by term integration of the binomial expansion . This gives p=9 for numerator and p=10 for denominator. Next to find 100 × 0 1 ( 1 x 10 ) 9 d x 0 1 ( 1 x 10 ) 10 d x 100 \times \dfrac{\int_0^1(1-x^{10})^9 \cdot dx}{\int_0^1(1-x^{10})^{10}\cdot dx} integrate the denominator by parts. 0 1 ( 1 x 10 ) 10 1 d x = ( 1 x 10 ) 10 x 0 1 0 1 10 ( 1 x 10 ) 9 ( 10 x 9 ) x d x \int_0^1(1-x^{10})^{10}\cdot1\cdot dx = (1-x^{10})^{10}\cdot x|_0^1-\int_0^110(1-x^{10})^9(-10x^9)\cdot xdx = 0 + 100 0 1 ( 1 x 10 ) 9 [ 1 ( 1 x 10 ) ] d x =0+100\int_0^1(1-x^{10})^9[1-(1-x^{10})]\cdot dx which upon transposing gives the result as 101.

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