Trigonometry, find the minimum of tan(a).tan(b).tan(c)

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Homework Help Overview

The problem involves finding the minimum value of the product of the tangents of three acute angles, given that their sum equals 180 degrees. Additionally, there is a second part that requires proving an inequality involving the sum of the tangents when the angles sum to 90 degrees.

Discussion Character

  • Exploratory, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants discuss expanding the tangent function and using relationships between angles. Some suggest employing geometric and arithmetic means, while others consider using Lagrange multipliers. There are also inquiries about attempts to solve the second question.

Discussion Status

Several participants have offered different approaches to tackle the first problem, including algebraic manipulations and geometric interpretations. There is a recognition of the need for more attempts to engage with the problems, particularly for the second question, but no consensus has been reached on a specific method.

Contextual Notes

Participants note that the angles are acute and that specific mathematical techniques, such as Lagrange multipliers, may be applicable. There is an emphasis on the importance of showing attempts in order to receive further assistance.

Michael_Light
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Homework Statement



1. Given a,b,c are acute angles and a + b + c =180. find the minimum of tan(a).tan(b).tan(c)

2. Prove that if a+b+c=90, then tan(a)+tan(b)+tan(c) >= 31/2

Homework Equations


The Attempt at a Solution



I don't even have any ideas how should i start to find/prove them... any hints?
 
Last edited:
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For the first one I'd try to expand the left side after taking the tangent of both sides.

tan(a+b+c) = tan(180°)
 
Following rock.freak's suggestion, I would write the tangent of the third angle in terms of the tangents of the other two angles, and would use the relation between geometric and arithmetic means.

ehild
 
tan(a)*tan(b)*tan(c) = -tan(b+c) *tan(b)*tan(c) = (tanb+tanc)/(1-tanb*tanc) *tanb*tanc
call S = tanb + tanc and P = tanb * tanc S^2>= 4P this should be easy from here on
 
Still cannot do... how bout question (2)? Any hints?
 
you can use the langrange function to do it, it is quite easy if u use it
 
We can not help if you do not show any attempt.

ehild
 
NeroKid said:
you can use the langrange function to do it, it is quite easy if u use it

Note that it is Precalculus Math.

ehild
 
then just have to expand them to the sum and the product which is pretty much easier to solve
 
  • #10
For the second one I would consider a right angled triangle and see if that can help.
 
  • #11
Michael_Light said:
1. Given a,b,c are acute angles and a + b + c =180. find the minimum of tan(a).tan(b).tan(c)
ehild said:
Following rock.freak's suggestion, I would write the tangent of the third angle in terms of the tangents of the other two angles, and would use the relation between geometric and arithmetic means.

ehild
NeroKid said:
tan(a)*tan(b)*tan(c) = -tan(b+c) *tan(b)*tan(c) = (tanb+tanc)/(1-tanb*tanc) *tanb*tanc
call S = tanb + tanc and P = tanb * tanc S^2>= 4P this should be easy from here on
Michael_Light said:
Still cannot do...

Michael, are you trying?

You need to minimise (tanb+tanc)/(1-tanb*tanc) *tanb*tanc …

surely you have some idea how to do that?​
 

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