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Difference between revisions of "2002 AMC 10A Problems/Problem 3"

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==Solution==
 
==Solution==
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The best way to solve this problem is by simple brute force. We find that the only other value is <math>(2^2)^{2^2}=4^{2^2}=4^4=256</math>. Our answer is just <math>\boxed{\text{(B)}\ 1}</math>.
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The best way to solve this problem is by simple brute force.
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It is convenient to drop the usual way how exponentiation is denoted, and to write the formula as <math>2\uparrow 2\uparrow 2\uparrow 2</math>, where <math>\uparrow</math> denotes exponentiation. We are now examining all ways to add parentheses to this expression. There are 5 ways to do so:
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# <math>2\uparrow (2\uparrow (2\uparrow 2))</math>
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# <math>2\uparrow ((2\uparrow 2)\uparrow 2)</math>
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# <math>((2\uparrow 2)\uparrow 2)\uparrow 2</math>
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# <math>(2\uparrow (2\uparrow 2))\uparrow 2</math>
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# <math>(2\uparrow 2)\uparrow (2\uparrow 2)</math>
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We can note that <math>2\uparrow (2\uparrow 2) = (2\uparrow 2)\uparrow 2 =16</math>. Therefore options 1 and 2 are equal, and options 3 and 4 are equal.
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Option 1 is the one given in the problem statement. Thus we only need to evaluate options 3 and 5.
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<math>((2\uparrow 2)\uparrow 2)\uparrow 2 = 16\uparrow 2 = 256</math>
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<math>(2\uparrow 2)\uparrow (2\uparrow 2) = 4 \uparrow 4 = 256</math>
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Thus the only other result is <math>256</math>, and our answer is <math>\boxed{\text{(B)}\ 1}</math>.
  
 
==See Also==
 
==See Also==

Revision as of 11:37, 24 January 2009

Problem

According to the standard convention for exponentiation,

$2^{2^{2^2}} = 2^{\left(2^{\left(2^2\right)}\right)} = 2^{16} = 65,536$.

If the order in which the exponentiations are performed is changed, how many other values are possible?


$\text{(A)}\ 0 \qquad \text{(B)}\ 1 \qquad \text{(C)}\ 2 \qquad \text{(D)}\ 3 \qquad \text{(E)}\ 4$

Solution

The best way to solve this problem is by simple brute force.

It is convenient to drop the usual way how exponentiation is denoted, and to write the formula as $2\uparrow 2\uparrow 2\uparrow 2$, where $\uparrow$ denotes exponentiation. We are now examining all ways to add parentheses to this expression. There are 5 ways to do so:

  1. $2\uparrow (2\uparrow (2\uparrow 2))$
  2. $2\uparrow ((2\uparrow 2)\uparrow 2)$
  3. $((2\uparrow 2)\uparrow 2)\uparrow 2$
  4. $(2\uparrow (2\uparrow 2))\uparrow 2$
  5. $(2\uparrow 2)\uparrow (2\uparrow 2)$

We can note that $2\uparrow (2\uparrow 2) = (2\uparrow 2)\uparrow 2 =16$. Therefore options 1 and 2 are equal, and options 3 and 4 are equal. Option 1 is the one given in the problem statement. Thus we only need to evaluate options 3 and 5.

$((2\uparrow 2)\uparrow 2)\uparrow 2 = 16\uparrow 2 = 256$

$(2\uparrow 2)\uparrow (2\uparrow 2) = 4 \uparrow 4 = 256$

Thus the only other result is $256$, and our answer is $\boxed{\text{(B)}\ 1}$.

See Also

2002 AMC 10A (Problems • Answer Key • Resources)
Preceded by
Problem 2 		Followed by
Problem 4
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25
All AMC 10 Problems and Solutions
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