2020 AMC 12B Problems/Problem 22
What is the maximum value of for real values of
We proceed by using AM-GM. We get . Thus, squaring gives us that . Rembering what we want to find(divide by ), we get the maximal values as , and we are done.
Set . Then the expression in the problem can be written as It is easy to see that is attained for some value of between and , thus the maximal value of is .
Solution 3 (Calculus Needed)
We want to maximize . We can use the first derivative test. Use quotient rule to get the following: Therefore, we plug this back into the original equation to get
First, substitute so that
When seen as a function, is a synthesis function that has as its inner function.
If we substitute , the given function becomes a quadratic function that has a maximum value of when .
Now we need to check if can have the value of in the range of real numbers.
In the range of (positive) real numbers, function is a continuous function whose value gets infinitely smaller as gets closer to 0 (as also diverges toward negative infinity in the same condition). When , , which is larger than .
Therefore, we can assume that equals to when is somewhere between 1 and 2 (at least), which means that the maximum value of is .
Let the maximum value of the function be . Then we have Solving for , we see We see that Therefore, the answer is .
Set , now we get and we want to find the maximum value of that expression. And that expression can be further simplified to , now lets set as and we get . Expand and we get . We can make it simpler by rearranging and getting as the original expression, so, now we need to find the minimum value of and take the negative of that and get our answer. So, we complete the square on to get it as . And by the trivial inequality, any real number squared is at least , so, to minimize this expression, we just set the square as and you subtract from to get as the minimum value of the expression and you take the negative of that to get as the final answer.
Problem starts at 2:10 in this video: https://www.youtube.com/watch?v=5HRSzpdJaX0
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