The question provided in the image involves trigonometric identities and a geometric classification based on angle values for triangle ABC. Let me break it down and solve it:

We are given that $A$, $B$, and $C$ are non-zero acute angles, and the two conditions are:

1. $\sum \left( \frac{\tan A}{\tan B \tan C} \right) = \sum (\tan A) - 2 \sum (\cot A)$
2. The minimum value of $\sum \left( \frac{\tan^2 A}{2} + \cot A \right)$ exists.

Step 1: Analyzing the Trigonometric Identity

The equation provided involving the sum of tangents and cotangents seems to lead towards a special type of triangle. The equation hints at symmetry in the trigonometric functions, which is often true in an equilateral triangle.

For an equilateral triangle, all angles are equal, meaning $A = B = C = 60^\circ$. This symmetry is what might cause the equation to hold.

Step 2: Minimizing the Function

The second condition asks us to find the minimum value of $\sum \left( \frac{\tan^2 A}{2} + \cot A \right)$. For acute angles, the expression will likely attain its minimum value when $A = B = C = 60^\circ$, which also supports the idea that the angles form an equilateral triangle.

Conclusion:

Given that the minimum value exists when $A = B = C$, the triangle formed by $A$, $B$, and $C$ is most likely an equilateral triangle.

Thus, the correct answer is: (c) equilateral triangle.

Would you like a detailed breakdown of any specific part of the solution or further explanation on the trigonometric identities used?

Here are 5 related questions for further practice:

1. What are the properties of an equilateral triangle related to angles and sides?
2. How can you derive the formula for the sum of angles in any triangle using trigonometric identities?
3. What is the minimum value of $\tan A + \tan B + \tan C$ for any triangle?
4. How does symmetry in trigonometric functions simplify solving geometry problems?
5. How would you prove the identity $\sum \tan A = \sum \cot A$ for a right-angled triangle?

Tip: Trigonometric identities often lead to elegant solutions in geometry, especially when angles are symmetrical.