Skill: Identify triangles

To identify a triangle, check its boundary:

1. It is closed—the sides connect all the way around.
2. It has exactly 3 straight sides.
3. It has exactly 3 vertices (corners).
4. It has exactly 3 angles.

The triangle can be large, small, tilted, or have sides of different lengths.

Example

Is this shape a triangle?

        A
       / \
      /   \
     B-----C

Step 1: Check that it is closed.
The sides connect from $A$ to $B$, from $B$ to $C$, and from $C$ back to $A$. There is no gap.

Step 2: Count the straight sides.
The shape has 3 straight sides: $\overline{AB}$, $\overline{BC}$, and $\overline{CA}$.

Step 3: Count the vertices.
The corners are $A$, $B$, and $C$, so there are 3 vertices.

Conclusion:
Yes, the shape is a triangle because it is closed and has exactly 3 straight sides, 3 vertices, and 3 angles.