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Calculus Level 5

x 2 2 × 3 + 2 x 3 3 × 4 + 3 x 4 4 × 5 + \frac {x^2}{2 \times 3} + \frac {2x^3}{3 \times 4} + \frac {3x^4}{4 \times 5} + \ldots

For x < 1 |x| < 1 , the series above converge to ( p q x ) ln ( 1 x ) q \left (p - \frac q x \right) \ln (1 - x) - q .

Find the value of p 4 + q 4 p^4 + q^4 .


The answer is 17.

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2 solutions

Aareyan Manzoor
Feb 25, 2015

first, in summation form n = 1 ( ( n 1 ) x n n ( n + 1 ) ) \sum_{n=1}^\infty (\dfrac{(n-1)x^{n}}{n(n+1)}) now, factor = ( n 1 ) x n n ( n + 1 ) = n x n n ( n + 1 ) x n n ( n + 1 ) = x n n + 1 x n ( 1 n ( n + 1 ) ) = x n n + 1 x n ( 1 n 1 n + 1 ) = 2 x n n + 1 x n n \begin{array}{c}=&\dfrac{(n-1)x^{n}}{n(n+1)}\\ =&\dfrac{nx^{n}}{n(n+1)}-\dfrac{x^{n}}{n(n+1)}\\ =&\dfrac{x^{n}}{n+1}-x^n(\dfrac{1}{n(n+1)})\\ =&\dfrac{x^{n}}{n+1}-x^n(\dfrac{1}{n}-\dfrac{1}{n+1})\\ =&2\dfrac{x^{n}}{n+1}-\dfrac{x^n}{n} \end{array} hence n = 1 ( ( n 1 ) x n n ( n + 1 ) ) = 2 n = 1 ( x n n + 1 ) n = 1 ( x n n ) \sum_{n=1}^\infty (\dfrac{(n-1)x^{n}}{n(n+1)})=2\sum_{n=1}^\infty (\dfrac{x^{n}}{n+1})-\sum_{n=1}^\infty(\dfrac{x^n}{n}) by a Maclaurin series expansions, n = 1 ( x n n ) = log ( 1 x ) \sum_{n=1}^\infty(\dfrac{x^n}{n})=-\log (1-x) and = n = 1 ( x n n + 1 ) = x 2 + x 2 3 + x 3 4 . . . . . . = x 2 2 + x 3 3 + x 4 4 . . . x = x + x 2 2 + x 3 3 + x 4 4 . . . . . x x = n = 1 ( x n n ) x 1 = log ( 1 x ) x 1 \begin{array}{c}=&\sum_{n=1}^\infty(\dfrac{x^n}{n+1})\\=&\dfrac{x}{2}+\dfrac{x^2}{3}+\dfrac{x^3}{4}......\\=&\dfrac{\dfrac{x^2}{2}+\dfrac{x^3}{3}+\dfrac{x^4}{4}...}{x}\\=&\dfrac{x+\dfrac{x^2}{2}+\dfrac{x^3}{3}+\dfrac{x^4}{4}.....-x}{x}\\=&\dfrac{\sum_{n=1}^\infty(\dfrac{x^n}{n})}{x}-1\\=&-\dfrac{\log (1-x)}{x}-1\end{array} hence 2 n = 1 ( x n n + 1 ) n = 1 ( x n n ) = 2 ( log ( 1 x ) x 1 ) + log ( 1 x ) = ( 1 2 x ) log ( 1 x ) 2 2\sum_{n=1}^\infty (\dfrac{x^{n}}{n+1})-\sum_{n=1}^\infty(\dfrac{x^n}{n})=2(-\dfrac{\log (1-x)}{x}-1)+\log (1-x)=(1-\dfrac{2}{x})\log (1-x)-2 and 1 4 + 2 4 = 17 1^4+2^4=\boxed{17} . the question must mention that x < 1 |x|<1 and i dont see what is so algebric 'bout it, more of a calculus problem.

Thanks. I've edited the question accordingly.

Calvin Lin Staff - 6 years, 3 months ago
Kartik Sharma
Feb 1, 2015

Consider the series -

1 + x + x 2 + x 3 + x 4 + . . . . . . . = 1 1 x 1 + x + {x}^{2} + {x}^{3} + {x}^{4} +....... = \frac{1}{1-x}

Differentiating and multiplying by x x ,

x + 2 x 2 + 3 x 3 + 4 x 4 + . . . . . = x ( 1 x ) 2 x + 2{x}^{2} + 3{x}^{3} + 4{x}^{4} +..... = \frac{-x}{{(1-x)}^{2}}

Integrating,

x 2 2 + 2 x 3 3 + . . . . = x ( 1 x ) 2 \frac{{x}^{2}}{2} + \frac{2{x}^{3}}{3} + .... = \int{\frac{-x}{{(1-x)}^{2}}}

Using u-substitution as u = 1 x u = 1 -x ,

x 2 2 + 2 x 3 3 + . . . . = 1 u u 2 \frac{{x}^{2}}{2} + \frac{2{x}^{3}}{3} + .... = \int{\frac{1 - u}{{u}^{2}}}

= 1 u 2 1 u =\int{\frac{1}{{u}^{2}}} - \int{\frac{1}{u}}

x 2 2 + 2 x 3 3 + . . . . = ( 1 1 x + l n ( 1 x ) ) \frac{{x}^{2}}{2} + \frac{2{x}^{3}}{3} + .... = -(\frac{1}{1-x} + ln(1-x))

Again integrating,

x 3 3.2 + 2 x 4 4.3 + . . . . = 1 1 x l n ( 1 x ) \frac{{x}^{3}}{3.2} + \frac{2{x}^{4}}{4.3} + .... = -\int{\frac{1}{1-x}} - \int{ln(1-x)}

Once again using u-substitution and IBP,

x 3 3.2 + 2 x 4 4.3 + . . . . = l n ( 1 x ) + ( 1 x ) l n ( 1 x ) + x \frac{{x}^{3}}{3.2} + \frac{2{x}^{4}}{4.3} + .... = ln(1-x) + (1-x)ln(1-x) + x

x 3 3.2 + 2 x 4 4.3 + . . . . = ( 2 x ) l n ( 1 x ) + x \frac{{x}^{3}}{3.2} + \frac{2{x}^{4}}{4.3} + .... = (2-x)ln(1-x) + x

Dividing by x,

x 2 3.2 + 2 x 3 4.3 + . . . . . = ( 2 x 1 ) l n ( 1 x ) + 1 \frac{{x}^{2}}{3.2} + \frac{2{x}^{3}}{4.3}+..... = (\frac{2}{x} - 1)ln(1-x) + 1

p 4 + q 4 = 2 4 + 1 4 = 17 {p}^{4} + {q}^{4} = {2}^{4} + {1}^{4} = 17

I did the same, but how is the ans you got equal to ( 1 2 x ) ( l n ( 1 x ) ) 2 (1-\frac{2}{x})(ln(1-x))-2

Keshav Tiwari - 5 years, 10 months ago

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@Kartik Sharma

Keshav Tiwari - 5 years, 9 months ago

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