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

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SUMMARY

The discussion focuses on finding the minimum value of the product tan(a)·tan(b)·tan(c) for acute angles a, b, and c, where a + b + c = 180°. Participants suggest using the relationship between the tangents of the angles and the geometric and arithmetic means to solve the problem. Additionally, the second question involves proving that tan(a) + tan(b) + tan(c) ≥ √3 when a + b + c = 90°, with hints to utilize the Lagrange multiplier method for optimization. The conversation emphasizes the importance of demonstrating initial attempts to solve the problems.

PREREQUISITES
  • Understanding of trigonometric functions, specifically tangent.
  • Familiarity with the properties of acute angles in triangles.
  • Knowledge of the geometric and arithmetic mean inequalities.
  • Basic understanding of optimization techniques, including Lagrange multipliers.
NEXT STEPS
  • Study the relationship between the tangent of angles in a triangle.
  • Learn about the geometric and arithmetic mean inequalities in detail.
  • Research the application of Lagrange multipliers in optimization problems.
  • Explore trigonometric identities and their applications in solving angle-related problems.
USEFUL FOR

Students studying precalculus mathematics, particularly those interested in trigonometry and optimization techniques, as well as educators seeking to enhance their teaching methods in these topics.

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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