2. Triangles
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3. Triangle:
• A closed, three straight-sided shape is called a
triangle.

4. Naming triangles
• Triangles are named according to the lengths
of their sides, or the size of their angles.

5. Naming triangles according to the
lengths of their sides
• Equilateral triangle:
All three sides are equal in length. (Also all three
angles are equal in size.)

6. Naming triangles according to the
lengths of their sides
• Isosceles triangle:
Two sides are equal in length. (Also two angles
at the base are equal in size.)

7. Naming triangles according to the
lengths of their sides
• Scalene triangle:
No sides are equal in length. (Also no angles are
equal in size.)

8. Naming triangles according to the size
of their angles
• Acute triangle:
All angles less than 90°.

9. Naming triangles according to the size
of their angles
• Right triangle:
Contains a right angle (90°).

10. Naming triangles according to the size
of their angles
• Obtuse triangle:
Contains an obtuse angle (greater than 90°).