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Correct answer: 8
We need to minimize
We will rewrite the expression in a form where AM-GM can be applied neatly.
1. Group the terms
Notice that and are natural pairs, because their exponents can be balanced.
Also,
So,
2. Apply AM-GM to each pair
For positive numbers, AM-GM gives
Pair 1:
Pair 2:
Thus,
This is true, but not immediately enough to get a clean minimum. So let us instead look for a point where all terms become simple and check whether AM-GM can directly produce a constant lower bound.
3. Test the natural equality point
At ,
So the minimum is at most .
Now we prove that for all .
4. Prove the lower bound
Using AM-GM on suitable grouped terms:
First group:
This alone is not enough. A better grouping is to compare each term with 1 through AM-GM:
For positive ,
Apply to and :
Apply to and :
So
Now at , this becomes , and the equality conditions of both AM-GM inequalities require
Hence is the common equality point.
To ensure it is indeed the minimum, define
Differentiate:
At ,
Second derivative:
So is strictly convex on , and the critical point is the unique global minimum.
Therefore,
5. Compare with stored answer
Stored correct answer:
Our derived answer is also , so they agree.
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