Scrambled Summations Reupload

Calculus Level 2

Evaluate when x = 0.5 x = 0.5 : 1 + n = 1 x n ( 1 ( n 1 ) ! ( 1 ) n ) e t where t = ln ( 10 ) k = 1 n log ( k ) \large 1+\sum_{n=1}^{\infty} \frac{x^n(1 - (n-1)!(-1)^n)}{e^t} \text{ where }t=\ln(10)\sum_{k=1}^{n}\log( k)

Notations:

  • log ( x ) = log 10 ( x ) \log(x)=\log_{10}(x)
  • x ! x! denotes x x factorial which can be expressed as x ( x 1 ) ( x 2 ) 2 1 x(x-1)(x-2)\dots 2 \cdot 1 for positive integer values of x x or 0 t x e t d t \displaystyle \int_{0}^{\infty}t^x e^{-t}\;dt for complex values of x x except for non-positive integer values


The answer is 2.05418637881.

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1 solution

James Watson
Aug 19, 2020

First, let us focus on t t . By revising our logarithm properties, we know that ln ( 10 ) log ( k ) = ln ( k ) \ln(10)\cdot\log(k) = \ln(k) because log a ( b ) log b ( c ) = log a ( c ) \log_a(b)\cdot \log_b(c)= \log_a(c) , so t t can be rewritten as t = k = 1 n ln ( 10 ) log ( k ) = k = 1 n ln ( k ) = ln ( n ! ) \begin{aligned} t &= \sum_{k=1}^{n}\ln(10)\log(k) \\ &= \sum_{k=1}^{n}\ln(k) \\ &= \ln(n!) \end{aligned} Now, we can plug t t into our main calculation: 1 + n = 1 x n ( 1 ( n 1 ) ! ( 1 ) n ) n ! = 1 + n = 1 x n x n ( n 1 ) ! ( 1 ) n ) n ! = 1 + n = 1 x n n ! x n ( n 1 ) ! ( 1 ) n n ( n 1 ) ! = 1 + n = 1 x n n ! m = 1 x m ( 1 ) m m = n = 0 x n n ! m = 1 x m ( 1 ) m m \begin{aligned} 1+\sum_{n=1}^{\infty}\frac{x^n(1-(n-1)!(-1)^n)}{n!} &= 1+\sum_{n=1}^{\infty}\frac{x^n-x^n(n-1)!(-1)^n)}{n!} \\ &= 1+\sum_{n=1}^{\infty}\frac{x^n}{n!}-\frac{x^n\sout{(n-1)!}(-1)^n}{n\sout{(n-1)!}} \\ &= 1+\sum_{n=1}^{\infty}\frac{x^n}{n!} - \sum_{m=1}^{\infty}\frac{x^m(-1)^m}{m} \\ &= \sum_{n=0}^{\infty}\frac{x^n}{n!} - \sum_{m=1}^{\infty}\frac{x^m(-1)^m}{m} \end{aligned} We know that the Maclaurin Series of e x e^x is n = 0 x n n ! \displaystyle \sum_{n=0}^{\infty}\frac{x^n}{n!} and for ln ( 1 + x ) \ln(1+x) is m = 1 x m ( 1 ) m m \displaystyle - \sum_{m=1}^{\infty}\frac{x^m(-1)^m}{m} for x < 1 |x| < 1 , so our main calculation shrinks down to e x + ln ( 1 + x ) e^x + \ln(1+x)

Now we can plug in x = 0.5 x=0.5 and we see that the answer is e 0.5 + ln ( 1.5 ) = 2.05418637881 \large e^{0.5} + \ln(1.5) = \green{\boxed{2.05418637881\dots}}

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