This problem involves expressing the sum of the angles in a triangle, which is a fundamental concept in geometry.

Step-by-Step Explanation:

1. Understanding the Triangle: The triangle has three angles labeled as $\angle 1$, $\angle 2$, and $\angle 3$. Their measures are represented as $m\angle 1$, $m\angle 2$, and $m\angle 3$, respectively.

2. Key Concept: In any triangle, the sum of the measures of the three angles is always $180^\circ$. This can be written as: $m\angle 1 + m\angle 2 + m\angle 3 = 180^\circ$

3. Writing the Expression: The expression for the sum of the angles is simply: $m\angle 1 + m\angle 2 + m\angle 3$

4. Inputting the Expression: Using the given buttons, click $m\angle 1$, the addition sign ($+$), $m\angle 2$, another addition sign ($+$), and $m\angle 3$ to create the expression: $m\angle 1 + m\angle 2 + m\angle 3$

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5 Related Questions:

1. Why is the sum of the angles in a triangle always $180^\circ$?
2. How would the sum of the angles change in a quadrilateral or polygon?
3. Can the sum of the angles in a triangle ever exceed $180^\circ$? Why or why not?
4. How can you use this property of triangles to find a missing angle if two angles are known?
5. How does this principle apply to triangles in non-Euclidean geometry?

Tip:

Always remember that the sum of the interior angles in a triangle is $180^\circ$, regardless of the type of triangle (scalene, isosceles, or equilateral). This rule is a cornerstone of Euclidean geometry!