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Algebra Difficulty 3.6 AMC 10/12 Find the answer

Given a>0a \gt 0, b>0b \gt 0, and a+2b=2a+2b=2, find the minimum value of a+bab\frac{{a+b}}{{ab}}.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

To find the minimum value of a+bab\frac{a+b}{ab} given a>0a > 0, b>0b > 0, and a+2b=2a+2b=2, we proceed as follows:

1. First, we express a+bab\frac{a+b}{ab} in terms of the inverses of aa and bb:

a+bab=1a+1b \frac{a+b}{ab} = \frac{1}{a} + \frac{1}{b}

2. Recognizing that a+2b=2a+2b=2, we can multiply the sum of inverses by a clever form of 11 (which is a+2b2\frac{a+2b}{2}) to facilitate the use of the AM-GM inequality:

1a+1b=12(1a+1b)(a+2b) \frac{1}{a} + \frac{1}{b} = \frac{1}{2}\left(\frac{1}{a} + \frac{1}{b}\right)(a+2b)

3. Simplify the expression by distributing and rearranging terms:

=12(3+2ba+ab) = \frac{1}{2}\left(3 + \frac{2b}{a} + \frac{a}{b}\right)

4. Apply the AM-GM inequality, xyx+y2\sqrt{xy} \le \frac{x+y}{2}, to the terms 2ba\frac{2b}{a} and ab\frac{a}{b}:

12(3+2ba+ab)12(3+22baab) \frac{1}{2}\left(3 + \frac{2b}{a} + \frac{a}{b}\right) \ge \frac{1}{2}\left(3 + 2\sqrt{\frac{2b}{a} \cdot \frac{a}{b}}\right)

5. Simplify the expression under the square root and the inequality:

=12(3+22) = \frac{1}{2}\left(3 + 2\sqrt{2}\right)

6. The equality holds (which means we have found the minimum) if and only if 2ba=ab\frac{2b}{a} = \frac{a}{b}, or equivalently a2=2b2a^2 = 2b^2. Solving this together with a+2b=2a+2b=2 gives b=22b = 2 - \sqrt{2} and a=222a = 2\sqrt{2} - 2.

Therefore, the minimum value of a+bab\frac{a+b}{ab}, given the conditions, is 12(3+22)\boxed{\frac{1}{2}(3+2\sqrt{2})}.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.