Complex exponential factorials!

Algebra Level 5

x ! ( x × x 3 4 ) x ! = ( x ! ( i 2 ) x ! ) x 8820 \LARGE \sqrt[{\left(x × \sqrt[4]{x^3}\right)}^{x!}]{x!} = {\left(\sqrt[{\left(i^2\right)}^{x!}]{x!}\right)}^{x^{-8820}}

Find the real value of x x satisfying the real equation above.

Note :- Here x ∉ { 1 , 0 , 1 } x \not\in \{-1 , 0 , 1\} .


Why not have Fun with exponents ?


The answer is 7.

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1 solution

Ashish Menon
May 15, 2016

x ! ( x × x 3 4 ) x ! = ( x ! ( i 2 ) x ! ) x 8820 x ! x 7 4 x ! = ( x ! 1 x ! ) x 8820 x ! 1 x 7 4 x ! = x ! x 8820 x ! x 7 4 x ! = x ! x 8820 Equating the powers, we get : x 7 4 x ! = x 8820 Equating the powers, we get : 7 4 x ! = 8820 x ! = 8820 × 4 7 x ! = 5040 x ! = 7 × 6 × 5 × 4 × 3 × 2 × 1 x = 7 \begin{aligned} \LARGE \sqrt[{\left(x × \sqrt[4]{x^3}\right)}^{x!}]{x!} & = \LARGE {\left(\sqrt[{\left(i^2\right)}^{x!}]{x!}\right)}^{x^{-8820}}\\ \\ \LARGE \sqrt[x^{\frac{7}{4}x!}]{x!} & = \LARGE {\left(\sqrt[{-1}^{x!}]{x!}\right)}^{x^{-8820}}\\ \\ \LARGE {x!}^{\tfrac{1}{x^{\frac{7}{4}x!}}} & = \LARGE {x!}^{x^{-8820}}\\ \\ \LARGE {x!}^{x^{-\frac{7}{4}x!}} & = \LARGE {x!}^{x^{-8820}}\\ \\ \text{Equating the powers, we get}:-\\ \LARGE x^{-\tfrac{7}{4}x!} & = \LARGE x^{-8820}\\ \\ \text{Equating the powers, we get}:-\\ \Large {\cancel{-}\dfrac{7}{4}x!} & = \Large {\cancel{-}8820}\\ \\ \Large x! & = \Large \dfrac{8820 × 4}{7}\\ \\ \Large x! & = \Large 5040\\ \\ \Large x! & = \Large 7×6×5×4×3×2×1\\ \\ \Large x & = \Large \boxed{7} \end{aligned}

In step 2, i x ! = 1 i^{x!} = 1 , because x! is even for all x ∉ { 1 , 0 , 1 } x \not\in \{-1 , 0 , 1\} because they are multiple of 2 2 .

i 2 = 1 1 i^{2} = -1 \ne 1

A Former Brilliant Member - 5 years, 1 month ago

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Oh yeah thanks, but answer remains same as explained above.

Ashish Menon - 5 years, 1 month ago

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nice Solution+Question .. +1

Sabhrant Sachan - 5 years, 1 month ago

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@Sabhrant Sachan The reason I post these exponent questions is because of wncouragement of people like you :)

Ashish Menon - 5 years, 1 month ago

@Sabhrant Sachan Jeez, Thanks!

Ashish Menon - 5 years, 1 month ago

wait... How did you get x 4 × x 3 = x 7 4 x^4\times \sqrt{x^3} = x^{\frac74}

x 4 × x 3 x 4 × x 3 2 x 4 + 3 2 x 11 2 x^4\times \sqrt{x^3} \implies x^4\times x^{\frac32} \implies x^{4+\frac32} \implies x^{\frac{11}2}

Sabhrant Sachan - 5 years, 1 month ago

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@Sabhrant Sachan It is x × x 3 4 x × \sqrt[4]{x^3} wait I qill clear it up in a jiffy.

Ashish Menon - 5 years, 1 month ago

You should mention in the question that x is positive Natural Number because for x = 1/2, (-1)^x! becomes a number other than 1.

Vaibhav Priyadarshi - 2 years, 11 months ago

May I ask for some tips on how do you know that 7 ! = 5040 7!=5040 without using calculator? Or is it impossible? Teehee. 😅

Nanda Rahsyad - 5 years, 1 month ago

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if you practice enough questions, you will remember factorial of numbers till 8 or 10

Sabhrant Sachan - 5 years, 1 month ago

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Correct! (+1)

Ashish Menon - 5 years, 1 month ago

I have remembered factprials upto 10 XD

Ashish Menon - 5 years, 1 month ago

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Wow... impressive XD

Nanda Rahsyad - 5 years, 1 month ago

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@Nanda Rahsyad Thank you :)

Ashish Menon - 5 years, 1 month ago

Why dont you update your score in my set :) You have solved this correctly (+1)

Ashish Menon - 5 years, 1 month ago

Factorize 5040.

Vaibhav Priyadarshi - 2 years, 11 months ago

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