2021 AMC 12B Problems/Problem 21
Let be the sum of all positive real numbers for whichWhich of the following statements is true?
Note that (At this point we see by inspection that is a solution.)
We simplify the RHS, then take the base- logarithm for both sides: The RHS is a line; the 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
We rewrite the right side without using square roots, then take the base- logarithm for both sides: By observations, is one solution. Graphing and we conclude 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
The graphs of and are shown below: ~MRENTHUSIASM
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 .
Video Solution by OmegaLearn (Logarithmic Tricks)
Video Solution by hippopotamus1
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