Solve 1/a + 1/b + 1/c When a+b+c+2=abc

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Discussion Overview

The discussion revolves around the problem of finding the smallest value of \( \frac{1}{a} + \frac{1}{b} + \frac{1}{c} \) given the constraint \( a + b + c + 2 = abc \), where \( a, b, c \) are positive real numbers. The scope includes mathematical reasoning and exploration of potential solutions.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant suggests that if \( a, b, c \) are all equal to 2, then the condition holds and the value of \( \frac{1}{a} + \frac{1}{b} + \frac{1}{c} \) is 1.5.
  • Another participant agrees with the previous calculation but raises the concern that there may be other combinations of \( a, b, c \) that yield a value less than 1.5.
  • A different approach is proposed involving Lagrange multipliers to minimize the function under the given constraint, although it is noted that this method may be complex.
  • Another participant suggests squaring terms or using the AM-GM inequality as potential strategies, indicating that the problem might be typical of high school math competitions.
  • A specific example is provided where setting \( b = c = 3 \) leads to another solution of \( (1, 3, 3) \), although it is acknowledged that this does not necessarily provide a better solution.
  • There is a mention of the symmetric nature of the problem, suggesting that symmetric solutions are often optimal.

Areas of Agreement / Disagreement

Participants express differing views on whether the solution of 1.5 is indeed the minimum, with some suggesting the possibility of lower values existing. The discussion remains unresolved regarding the optimality of the proposed solutions.

Contextual Notes

There are indications of missing assumptions and the need for further exploration of the mathematical steps involved in proving or disproving the minimum value. The discussion also highlights the potential complexity of the problem.

Hobold
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A friend of mine gave me this problem to solve because he couldn't and I've been stuck in it for some time and also can't solve.

If a, b, c are positive real numbers and a + b + c + 2 = abc, find the smallest value of 1/a + 1/b + 1/c.

PS: I'm guessing this is the right section, 'cause this is not a homework.
 
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The only thing I can see that fits for the first part is a, b, and c are all 2.
2+2+2+2=8 and 2x2x2=8. So the last part would be 1.5
smallest value of 1/2 + 1/2 + 1/2=1.5
Unless I am reading something wrong.
 
physi-man said:
The only thing I can see that fits for the first part is a, b, and c are all 2.
2+2+2+2=8 and 2x2x2=8. So the last part would be 1.5
smallest value of 1/2 + 1/2 + 1/2=1.5
Unless I am reading something wrong.

Yep, I got this result as well, but there's nothing that proves that there isn't a combination of numbers which will make 1/a + 1/b + 1/c < 1.5
 
There are two ways that you can do this:
1) Use Lagrange multipliers. You're trying to minimize a function given a constraint so it would work, but might be ugly.

2) Square something crazy. This looks like it's probably a high school math competition problem, which means that 1 is an inappropriate solution. Instead you probably need to do something like square something, which you know is greater than or equal to zero, or maybe use AM/GM. I don't see exactly what it is you would be using but I'll think about it

As for other solutions:

a(1-bc)+b+c+2=0

(-2-b-c)/(1-bc)=a. Now pick values of b and c making this positive. For example if b=c=3, we get (-2-6)/(1-9)=-8/-8=1. So (1,3,3) is also a solution

Of course this isn't a better solution, but you never know if there is one. The symmetric solution is usually optimal when the problem is symmetric.
 
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