2021 AMC 12B Problems/Problem 21
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
(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
Solution 3 (Cleaner Solution by Graphing and Very 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 that Finally, we conclude that and the answer must be
Graphs of and in Desmos: https://www.desmos.com/calculator/rjlgnikxaz
Video Solution by OmegaLearn (Logarithmic Tricks)
Video Solution by hippopotamus1:
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