Difference between revisions of "2021 AMC 12B Problems/Problem 21"
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<math>\textbf{(A) }S<\sqrt2 \qquad \textbf{(B) }S=\sqrt2 \qquad \textbf{(C) }\sqrt2<S<2\qquad \textbf{(D) }2\le S<6 \qquad \textbf{(E) }S\ge 6</math> | <math>\textbf{(A) }S<\sqrt2 \qquad \textbf{(B) }S=\sqrt2 \qquad \textbf{(C) }\sqrt2<S<2\qquad \textbf{(D) }2\le S<6 \qquad \textbf{(E) }S\ge 6</math> | ||
+ | |||
+ | == Solution 1 (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 <math>x=\sqrt{2}</math>. From this, we can deduce that this equality has two solutions, since <math>\sqrt{2}^{2^{x}}</math> grows faster than <math>x^{2^{\sqrt{2}}}</math> (for greater values of <math>x</math>) and <math>\sqrt{2}^{2^{x}}</math> is greater than <math>x^{2^{\sqrt{2}}}</math> for <math>x<\sqrt{2}</math> and less than <math>x^{2^{\sqrt{2}}}</math> for <math>\sqrt{2}<x<n</math>, where <math>n</math> is the second solution. Thus, the answer cannot be <math>\text{A}</math> or <math>\text{B}</math>. We then start plugging in numbers to roughly approximate the answer. When <math>x=2</math>, <math>x^{2^{\sqrt{2}}}>\sqrt{2}^{2^{x}}</math>, thus the answer cannot be <math>\text{C}</math>. Then, when <math>x=4</math>, <math>x^{2^{\sqrt{2}}}=4^{2^{\sqrt{2}}}<64<\sqrt{2}^{2^{x}}=256</math>. Therefore, <math>S<4+\sqrt{2}<6</math>, so the answer is <math>\boxed{\textbf{(D) } 2 \le S < 6}</math>. ~Baolan | ||
+ | |||
+ | == Solution 2 == | ||
+ | <math>x^{2^{\sqrt{2}}} = {\sqrt{2}}^{2^x}</math> | ||
+ | |||
+ | <math>2^{\sqrt{2}} \log x = 2^{x} \log \sqrt{2}</math> (At this point we see by inspection that <math>x=\sqrt{2}</math> is a solution.) | ||
+ | |||
+ | <math>\sqrt{2} \log 2 + \log \log x = x \log 2 + \log \log \sqrt{2}</math> | ||
+ | |||
+ | <math>\sqrt{2} + \log_2 \log_2 x = x + \log_2 \log_2 \sqrt{2} = x -1.</math> | ||
+ | |||
+ | <math>\log_2 \log_2 x = x - 1 - \sqrt{2}.</math> | ||
+ | |||
+ | RHS is a line. LHS is a concave curve that looks like a logarithm and has <math>x</math> intercept at <math>(2,0).</math> | ||
+ | There are at most two solutions, one of which is <math>\sqrt{2}.</math> But note that at <math>x=2,</math> we have <math>\log_2 \log_2 (2) = 0 > 2 - 1 - \sqrt{2},</math> meaning that the log log curve is above the line, so it must intersect the line again at a point <math>x > 2.</math> Now we check <math>x=4</math> and see that <math>\log_2 \log_2 (4) = 1 < 4 - 1 - \sqrt{2},</math> which means at <math>x=4</math> the line is already above the log log curve. Thus, the second solution lies in the interval <math>(2,4).</math> | ||
+ | The answer is <math>\boxed{2 \leq S < 6}.</math> | ||
+ | |||
+ | ~ ccx09 | ||
+ | |||
+ | ==Solution 3 (Graphs and Light Approximations)== | ||
+ | We rewrite the right side, then take the base-<math>2</math> logarithm for both sides: | ||
+ | <cmath>\begin{align*} | ||
+ | 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}. \hspace{20mm} (*) | ||
+ | \end{align*}</cmath> | ||
+ | By observations, <math>x=\sqrt2</math> is one solution. By quick sketches of <math>f(x)=2^{\sqrt2}\log_2{x}</math> and <math>g(x)=2^{x-1},</math> we know that <math>(*)</math> has two solutions, with <math>x=\sqrt2</math> the smaller solution. We construct the following table of values: | ||
+ | <cmath>\begin{array}{c|c|c|c} | ||
+ | & & & \\ [-2ex] | ||
+ | \boldsymbol{x} & \boldsymbol{f(x)=2^{\sqrt2}\log_2{x}} & \boldsymbol{g(x)=2^{x-1}} & \textbf{Comparison} \\ [1ex] | ||
+ | \hline | ||
+ | & & & \\ [-1ex] | ||
+ | 1 & 0 & 1 & \\ [1ex] | ||
+ | \sqrt2 & 2^{\sqrt2-1} & 2^{\sqrt2-1} & f\left(\sqrt2\right)=g\left(\sqrt2\right) \\ [1ex] | ||
+ | 2 & 2^{\sqrt2} & 2 & f(2)>g(2) \\ [1ex] | ||
+ | 3 & 2^{\sqrt2}\log_2{3} & 2^2 & \\ [1ex] | ||
+ | 4 & 2^{\sqrt2+1} & 2^3 & f(4)<g(4) \\ [1ex] | ||
+ | \end{array}</cmath> | ||
+ | Let <math>x=t</math> be the larger solution. Since exponential functions outgrow logarithmic functions, we have <math>f(x)<g(x)</math> for all <math>x>t.</math> By the <b>Intermediate Value Theorem</b>, we get <math>t\in(2,4),</math> from which <cmath>S=\sqrt2+t\in\left(\sqrt2+2,\sqrt2+4\right).</cmath> | ||
+ | Finally, approximating with <math>\sqrt2\approx1.414</math> results in <math>\boxed{\textbf{(D) }2\le S<6}.</math> | ||
+ | |||
+ | Graphs of <math>f(x)</math> and <math>g(x)</math> in Desmos: https://www.desmos.com/calculator/xyxzsvjort | ||
+ | |||
+ | ~MRENTHUSIASM | ||
== Video Solution by OmegaLearn (Logarithmic Tricks) == | == Video Solution by OmegaLearn (Logarithmic Tricks) == | ||
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~ pi_is_3.14 | ~ pi_is_3.14 | ||
− | == Solution | + | ==Video Solution by hippopotamus1:== |
− | + | https://www.youtube.com/watch?v=GjO6C_qC13U&feature=youtu.be | |
− | |||
− | |||
==See Also== | ==See Also== | ||
{{AMC12 box|year=2021|ab=B|num-b=20|num-a=22}} | {{AMC12 box|year=2021|ab=B|num-b=20|num-a=22}} | ||
{{MAA Notice}} | {{MAA Notice}} |
Latest revision as of 01:53, 27 June 2021
Contents
Problem
Let be the sum of all positive real numbers for whichWhich of the following statements is true?
Solution 1 (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.)
RHS is a line. LHS 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 (Graphs and Light Approximations)
We rewrite the right side, then take the base- logarithm for both sides: By observations, is one solution. By quick sketches of and we know that has two solutions, with the smaller solution. We construct the following table of values: Let be the larger solution. Since exponential functions outgrow logarithmic functions, we have for all By the Intermediate Value Theorem, we get from which Finally, approximating with results in
Graphs of and in Desmos: https://www.desmos.com/calculator/xyxzsvjort
~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 |
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