Think it out

Calculus Level 3

Find the sum up to infinite terms

1 + 1 12 + 1 80 + 1 448 + 1 2304 + 1+\frac{1}{12}+\frac{1}{80}+\frac{1}{448}+ \frac{1}{2304} + \ldots

Also see

Coordinate Geometry


The answer is 1.098.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Tanishq Varshney
Feb 27, 2015

l n ( 1 + x 1 x ) = 2 x + 2 3 x 3 + 2 5 x 5 + 2 7 x 7 . . . . . . . . ln(\frac{1+x}{1-x})=2x+\frac{2}{3}x^{3}+\frac{2}{5}x^{5}+\frac{2}{7}x^{7}........

Replace x = 1 2 x=\frac{1}{2}

A n s w e r = l n 3 Answer=ln3

Jeffrey Robles
Apr 12, 2015

The series can be written as

n = 1 1 2 2 n 2 ( 2 n 1 ) 2 n = 1 1 2 2 n 1 ( 2 n 1 ) \displaystyle \sum_{n=1}^\infty \frac{1}{2^{2n-2}\cdot(2n-1)}\Rightarrow \displaystyle 2\sum_{n=1}^\infty \frac{1}{2^{2n-1}\cdot(2n-1)}

The latter expression resembles that of the Maclaurin series representation of tanh 1 ( x ) \tanh^{-1} (x) .

tanh 1 ( x ) = n = 1 x 2 n 1 ( 2 n 1 ) tanh 1 ( x 2 ) = n = 1 x 2 n 1 2 2 n 1 ( 2 n 1 ) \tanh^{-1} (x)=\displaystyle \sum_{n=1}^\infty \frac{x^{2n-1}}{(2n-1)} \\ \tanh^{-1} (\frac{x}{2})=\displaystyle \sum_{n=1}^\infty \frac{x^{2n-1}}{2^{2n-1}\cdot(2n-1)}

Hence, with x = 1 x=1

n = 1 1 2 2 n 2 ( 2 n 1 ) = 2 tanh 1 ( 0.5 ) 1.0986 \displaystyle \sum_{n=1}^\infty \frac{1}{2^{2n-2}\cdot(2n-1)}=2\tanh^{-1} (0.5)\approx \boxed{1.0986}

Note: By applying the properties of hyperbolic functions, it can be shown that tanh 1 ( 0.5 ) = ln 3 2 \tanh^{-1}(0.5)=\frac{\ln{3}}{2}

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...