Four Fours/81/Solutions/2

Puzzle: Four Fours: $81$

Using exactly $4$ instances of the number $4$, the task is to write an expression for $81$, using whatever arithmetical operations you consider necessary.

Solution

$81 = \paren {\dfrac 4 {. \dot 4} }^{\tfrac 4 {\sqrt 4} }$

Sources

• 1980: Angela Dunn: Mathematical Bafflers (revised ed.) ... (previous) ... (next): $1$. Say it with Letters: Algebraic Amusements: Four Fours