The sum of the lengths of the sides of a triangle is called its .............

Answer

To solve the question, "The sum of the lengths of the sides of a triangle is called its .............", we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Sides of a Triangle**: A triangle has three sides. Let's denote them as \( a \), \( b \), and \( c \).

2. **Understand the Concept of Sum**: The problem is asking for the sum of the lengths of these three sides. We need to add the lengths of side \( a \), side \( b \), and side \( c \).

3. **Perform the Addition**: The sum of the lengths of the sides can be expressed mathematically as:
\[ a + b + c \]

4. **Define the Term**: The term that describes the sum of the lengths of the sides of a triangle is known as the "perimeter".

5. **Final Answer**: Therefore, the answer to the question is:
\[ \text{The sum of the lengths of the sides of a triangle is called its perimeter.} \]