Skill: Classify triangles by angle type

To classify a triangle by its angle type, look at the measures of its three interior angles:

• Acute triangle: all three angles are less than $90^\circ$.
• Right triangle: exactly one angle is $90^\circ$.
• Obtuse triangle: exactly one angle is greater than $90^\circ$.

The angles in every triangle add to $180^\circ$.

Example: Classify a triangle with angles $55^\circ$, $90^\circ$, and $35^\circ$.

1. Check the angle measures:

   • $55^\circ$ is less than $90^\circ$.
   • $90^\circ$ is exactly $90^\circ$.
   • $35^\circ$ is less than $90^\circ$.

2. The triangle has exactly one $90^\circ$ angle.

3. Therefore, the triangle is a right triangle.

Check:

$$55^\circ+90^\circ+35^\circ=180^\circ$$