Ramanujan's nested radical is great! (4)

Calculus Level 5

Find the value of x x satisfying the equation below.

( 201 6 π 0 0 ! ) ( 201 6 π 1 1 ! ) ( 201 6 π 4 4 ! ) ( 201 6 π 5 5 ! ) ( 201 6 π 8 8 ! ) ( 201 6 π 9 9 ! ) ( ) ( 201 6 π 2 2 ! ) ( 201 6 π 3 3 ! ) ( 201 6 π 6 6 ! ) ( 201 6 π 7 7 ! ) ( 201 6 π 10 10 ! ) ( 201 6 π 11 11 ! ) ( ) = 201 6 x \large \dfrac{(\sqrt[0!]{2016^{\pi^{0}}})(\sqrt[1!]{2016^{\pi^{1}}})(\sqrt[4!]{2016^{\pi^{4}}})(\sqrt[5!]{2016^{\pi^{5}}})(\sqrt[8!]{2016^{\pi^{8}}})(\sqrt[9!]{2016^{\pi^{9}}})(\dots)}{(\sqrt[2!]{2016^{\pi^{2}}})(\sqrt[3!]{2016^{\pi^{3}}})(\sqrt[6!]{2016^{\pi^{6}}})(\sqrt[7!]{2016^{\pi^{7}}})(\sqrt[10!]{2016^{\pi^{10}}})(\sqrt[11!]{2016^{\pi^{11}}})(\dots)}=2016^{x}

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


The answer is -1.

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1 solution

Tommy Li
Jul 1, 2016

Rewrite the above expression :

( 201 6 π 0 0 ! ) ( 201 6 π 1 1 ! ) ( 201 6 π 4 4 ! ) ( 201 6 π 5 5 ! ) ( 201 6 π 8 8 ! ) ( 201 6 π 9 9 ! ) ( ) ( 201 6 π 2 2 ! ) ( 201 6 π 3 3 ! ) ( 201 6 π 6 6 ! ) ( 201 6 π 7 7 ! ) ( 201 6 π 10 10 ! ) ( 201 6 π 11 11 ! ) ( ) \large \dfrac{(\sqrt[0!]{2016^{\pi^{0}}})(\sqrt[1!]{2016^{\pi^{1}}})(\sqrt[4!]{2016^{\pi^{4}}})(\sqrt[5!]{2016^{\pi^{5}}})(\sqrt[8!]{2016^{\pi^{8}}})(\sqrt[9!]{2016^{\pi^{9}}})(\dots)}{(\sqrt[2!]{2016^{\pi^{2}}})(\sqrt[3!]{2016^{\pi^{3}}})(\sqrt[6!]{2016^{\pi^{6}}})(\sqrt[7!]{2016^{\pi^{7}}})(\sqrt[10!]{2016^{\pi^{10}}})(\sqrt[11!]{2016^{\pi^{11}}})(\dots)}

= exp 2016 ( 1 + π π 2 2 ! π 3 3 ! + π 4 4 ! + π 5 5 ! π 6 6 ! π 7 7 ! + ) =\large \exp_{2016} (1+\pi-\frac{\pi^{2}}{2!}-\frac{\pi^{3}}{3!}+\frac{\pi^{4}}{4!}+\frac{\pi^{5}}{5!}-\frac{\pi^{6}}{6!}-\frac{\pi^{7}}{7!}+\dots)

= exp 2016 ( sin ( π ) + cos ( π ) ) =\large \exp_{2016} (\sin(\pi)+\cos(\pi))

= exp 2016 ( 1 ) =\large \exp_{2016} (-1)

x = 1 \Rightarrow x=-1

Notation : exp A ( B ) \exp_A (B) denotes the exponential function A B A^B .


Consider the power series for cos ( x ) \cos(x) and sin ( x ) \sin(x) :

sin ( x ) = x x 3 3 ! + x 5 5 ! x 7 7 ! + \sin(x)=x-\frac{x^{3}}{3!}+\frac{x^{5}}{5!}-\frac{x^{7}}{7!}+\dots

sin ( π ) = π π 3 3 ! + π 5 5 ! π 7 7 ! + \sin(\pi)=\pi-\frac{\pi^{3}}{3!}+\frac{\pi^{5}}{5!}-\frac{\pi^{7}}{7!}+\dots

cos ( x ) = 1 x 2 2 ! + x 4 4 ! x 6 6 ! + \cos(x)=1-\frac{x^{2}}{2!}+\frac{x^{4}}{4!}-\frac{x^{6}}{6!}+\dots

cos ( π ) = 1 π 2 2 ! + π 4 4 ! π 6 6 ! + \cos(\pi)=1-\frac{\pi^{2}}{2!}+\frac{\pi^{4}}{4!}-\frac{\pi^{6}}{6!}+\dots

sin ( π ) + cos ( π ) = 1 + π π 2 2 ! π 3 3 ! + π 4 4 ! + π 5 5 ! π 6 6 ! π 7 7 ! + \sin(\pi)+\cos(\pi)=1+\pi-\frac{\pi^{2}}{2!}-\frac{\pi^{3}}{3!}+\frac{\pi^{4}}{4!}+\frac{\pi^{5}}{5!}-\frac{\pi^{6}}{6!}-\frac{\pi^{7}}{7!}+\dots

sin ( π ) + cos ( π ) = 0 1 = 1 \sin(\pi)+\cos(\pi)=0-1=-1

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