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Number Theory Level 1

Evaluate ( ( 2 2 ) ! ! ) ! \big((2^2)!!\big)! .


The answer is 40320.

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Ashish Menon by Ashish Menon , 20, India

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1 solution

Ashish Menon
Apr 16, 2016

( ( 2 2 ) ! ! ) ! = ( 4 ! ! ) ! = ( 4 × 2 ) ! = 8 ! = 40320 \begin{aligned} ((2^2)!!)! & = (4!!)!\\ & = (4 × 2)!\\ & = 8!\\ & = \boxed{40320} \end{aligned}

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Note to people: a!! = a•(a-2)•(a-4)•...•4•2 if even and a!! = a•(a-2)•(a-4)•...•3•1 if odd. This is called a 'bifactorial'.

Goh Choon Aik - 5 years, 1 month ago

That's what I said. 8!=40320

Ruth MacGown - 1 year ago

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