Weird exponent

Level 2

The value of x x that satisfies the equation

8 x x x = 4 8^{x^{x^{x^\cdots}}}=4

can be expressed as a b c \frac{a\sqrt{b}}{c} where a , b , a,b, and c c are positive integers, a a and c c are relatively prime, and b b is not the square of any prime. Find a b c abc .


The answer is 108.

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2 solutions

Hahn Lheem
Jan 20, 2014

We can easily show that x x x . . . = 2 3 x^{x^{x^{...}}}=\frac{2}{3} . Notice that this can be expressed as x 2 3 = 2 3 x^{\frac{2}{3}}=\frac{2}{3} . Raising both sides of the equation by 3 2 \frac{3}{2} , we get x = 2 2 3 3 = 2 6 9 x=\frac{2\sqrt{2}}{3\sqrt{3}}=\frac{2\sqrt{6}}{9} . We now know a , b , a,b, and c c , so simple multiplication gives us 108 \boxed{108} .

Vincent Huang
Jan 20, 2014

Because log 8 4 = 2 3 \log_8 4 =\frac{2}{3} we have x x = 2 3 x^{x\cdots}= \frac{2}{3} or x 2 3 = 2 3 x^{\frac{2}{3}} = \frac{2}{3} Solving, we find x = 2 6 9 x=\frac{2\sqrt{6}}{9} so a b c = 108 abc=108

Hm, I haven't heard from Zehao in a while...

Alex Segesta - 6 years, 11 months ago

Darn, I'm muted on FTW

Alex Segesta - 6 years, 11 months ago

Zehao get on hangouts (ik changed name)

Alex Segesta - 6 years, 11 months ago

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