Prove Triangle Inequality: $\sum_{cyc} \sin A$

In summary, the conversation revolves around proving the inequality that in any triangle, the sum of the sines of the angles minus the product of the sines of the angles is greater than or equal to the sum of the cubes of the sines of the angles. The speaker apologizes for posting a challenge without being certain of the solution and asks for help understanding the use of the Rearrangement Inequality in the proof. They acknowledge their mistake in posting in multiple forums and apologize for it. A suggested solution is also provided.
  • #1
lfdahl
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MHB
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Prove, that for any triangle:\[ \sum_{cyc}\sin A - \prod_{cyc}\sin A \ge \sum_{cyc}\sin^3 A \]
 
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  • #2
I am so sorry, that I have posted a challenge, the solution of which, I am not certain. My problem is the use of the Rearrangement Inequality in the proof. I have asked in the forum: ”Pre-University Math/trigonometry” (http://mathhelpboards.com/trigonometry-12/usage-rearrangement-inequality-trigonometric-expression-20998.html#post95174), and I am aware, that it is bad policy to post a challenge in one forum and ask for a partial solution of it in another on the MHB site. Again, I am very sorry about this. It won´t happen again.

Here is the suggested solution:
Division by $\sin A \sin B \sin C$:

\[\sum_{cyc}\frac{1}{\sin B \sin C}-1 \geq \sum_{cyc}\frac{\sin^2 A}{\sin B \sin C}=\sum_{cyc}\frac{1- \cos^2 A}{\sin B \sin C} \\\\ \Rightarrow \sum_{cyc}\frac{\cos^2 A}{\sin B \sin C} \geq 1\]

Now, here comes the moment, where the Rearrangement Inequality is applied:

\[\sum_{cyc} \frac{\cos^2A}{\sin B\sin C}\geq \sum_{cyc} \frac{\cos B \cos C}{\sin B\sin C} =\sum_{cyc}\cot B \cot C = 1.\]
 

Related to Prove Triangle Inequality: $\sum_{cyc} \sin A$

1. What is the Triangle Inequality for a triangle?

The Triangle Inequality states that the sum of any two sides of a triangle must be greater than the third side. Mathematically, for a triangle with sides A, B, and C, the Triangle Inequality can be written as A + B > C, B + C > A, and A + C > B.

2. How is the Triangle Inequality related to the sum of trigonometric functions in a triangle?

The Triangle Inequality can be proven using the sum of trigonometric functions in a triangle. Specifically, the sum of sine functions in a triangle, which is represented by $\sum_{cyc} \sin A$, can be used to prove the Triangle Inequality. This is because the sine function is always positive, and therefore the sum of sines in a triangle must also be positive, making it a useful tool for proving the Triangle Inequality.

3. What is the significance of proving the Triangle Inequality using $\sum_{cyc} \sin A$?

Proving the Triangle Inequality using $\sum_{cyc} \sin A$ allows us to not only understand the relationship between the sides of a triangle, but also the relationship between the angles. This method provides a deeper understanding of the properties of a triangle and can be applied in various mathematical and scientific contexts.

4. Is the Triangle Inequality true for all triangles?

Yes, the Triangle Inequality is true for all triangles. This is because it is a fundamental property of triangles and is inherent in their geometric structure. Therefore, it can be applied to any type of triangle, including equilateral, isosceles, and scalene triangles.

5. Can the Triangle Inequality be used to solve real-world problems?

Yes, the Triangle Inequality can be applied to solve real-world problems, particularly in fields such as engineering, physics, and navigation. For example, it can be used to determine the shortest distance between two points, or to ensure the stability of a structure by checking if the sides of a triangle satisfy the Triangle Inequality.

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