Difference between revisions of "2021 AMC 12B Problems/Problem 21"
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We rewrite the right side, then take the base-<math>2</math> logarithm for both sides: | We rewrite the right side, then take the base-<math>2</math> logarithm for both sides: | ||
<cmath>\begin{align*} | <cmath>\begin{align*} | ||
− | x^{2^{\sqrt2}}=\sqrt2^{2^x} | + | x^{2^{\sqrt2}}&=\left(2^\frac12\right)^{2^x} \\ |
+ | x^{2^{\sqrt2}}&=2^{\frac12\cdot2^x} \\ | ||
+ | x^{2^{\sqrt2}}&=2^{2^{x-1}} \\ | ||
+ | \log_2{\left(x^{2^{\sqrt2}}\right)}&=\log_2{\left(2^{2^{x-1}}\right)} \\ | ||
+ | 2^{\sqrt2}\log_2{x}&=2^{x-1}. | ||
\end{align*}</cmath> | \end{align*}</cmath> | ||
Revision as of 04:14, 21 February 2021
Contents
Problem
Let be the sum of all positive real numbers for whichWhich of the following statements is true?
Solution (Rough Approximation)
Note that this solution is not recommended unless you're running out of time.
Upon pure observation, it is obvious that one solution to this equality is . From this, we can deduce that this equality has two solutions, since grows faster than (for greater values of ) and is greater than for and less than for , where is the second solution. Thus, the answer cannot be or . We then start plugging in numbers to roughly approximate the answer. When , , thus the answer cannot be . Then, when , . Therefore, , so the answer is . ~Baolan
Solution 2
(At this point we see by inspection that is a solution.)
LHS is a line. RHS is a concave curve that looks like a logarithm and has intercept at There are at most two solutions, one of which is But note that at we have meaning that the log log curve is above the line, so it must intersect the line again at a point Now we check and see that which means at the line is already above the log log curve. Thus, the second solution lies in the interval The answer is
~ ccx09
Solution 3 (Cleaner Solution by Graphing)
We rewrite the right side, then take the base- logarithm for both sides:
~MRENTHUSIASM
Video Solution by OmegaLearn (Logarithmic Tricks)
~ pi_is_3.14
Video Solution by hippopotamus1:
https://www.youtube.com/watch?v=GjO6C_qC13U&feature=youtu.be
See Also
2021 AMC 12B (Problems • Answer Key • Resources) | |
Preceded by Problem 20 |
Followed by Problem 22 |
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 12 Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.