Let p ∈ Q be a rational number such that p > 1 and letq := pp − 1 .Prove that for all a, b ≥ 0,ab ≤ app + bqq
Question
Let p ∈ Q be a rational number such that p > 1 and let q := p^{p - 1}. Prove that for all a, b ≥ 0,
ab ≤ ap^{p} + bq^{q}.
Solution
To prove that for all a, b ≥ 0, ab ≤ app + bqq, we can start by rearranging the inequality.
First, let's substitute q with pp - 1:
ab ≤ app + b(pp - 1)(pp - 1)
Next, we can expand the right side of the inequality:
ab ≤ app + b(p^2p^2 - 2pp + 1)
Simplifying further:
ab ≤ app + bp^4 - 2bp^3 + b
Now, let's factor out p from the second and third terms:
ab ≤ app + bp^3(p - 2) + b
Since p > 1, we know that p - 2 < 0. Therefore, we can replace p - 2 with -(2 - p):
ab ≤ app + b(-(2 - p)p^3) + b
Simplifying again:
ab ≤ app - b(2 - p)p^3 + b
Now, let's factor out b from the second and third terms:
ab ≤ app - b(2 - p)p^3 + b
ab ≤ app + bp(2 - p)p^2
Finally, we can rewrite the right side of the inequality as a product:
ab ≤ app + b(p^2)(2 - p)p
Now, we can see that the right side of the inequality is a product of three non-negative terms: app, p^2, and (2 - p)p. Since a and b are also non-negative, the product of these terms will always be greater than or equal to ab.
Therefore, we have proven that for all a, b ≥ 0, ab ≤ app + bqq.
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