A triangle has three sides and three angles


The three angles always add to 180°


Equilateral, Isosceles and Scalene       


There are three special names given to triangles that tell how many sides (or angles) are equal.


            There can be 3, 2 or no equal sides/angles:


                            Equilateral Triangle


Three equal sides

Three equal angles, always 60°

                Isosceles Triangle


Two equal sides

        Two equal angles               

                Scalene Triangle


No equal sides

No equal angles




What Type of Angle?



Triangles can also have names that tell you what type of angle is inside:



                        Acute Triangle


All angles are less than 90°               

                        Right Triangle


Has a right angle (90°)               

                        Obtuse Triangle               

Has an angle more than 90°                 


Combining the Names

            Sometimes a triangle will have two names, for example:



Right Isosceles Triangle


Has a right angle (90°), and also two equal angles


Can you guess what the equal angles are?               


Play With It ...


Try dragging the points around and make different triangles:


You might also like to play with the Interactive Triangle.



The perimeter is the distance around the edge of the triangle: just add up the three sides:


The area is half of the base times height.

  • "b" is the distance along the base
  • "h" is the height (measured at right angles to the base)

Area = ½ × b × h


The formula works for all triangles.


Note: a simpler way of writing the formula is bh/2



Example: What is the area of this triangle?



(Note: 12 is the height, not the length of the left-hand side)



Height = h = 12


Base = b = 20


Area = ½ × b × h = ½ × 20 × 12 = 120               



The base can be any side, Just be sure the "height" is measured at right angles to the "base":

(Note: You can also calculate the area from the lengths of all three sides using Heron's Formula.)



Why is the Area "Half of bh"?


Imagine you "doubled" the triangle (flip it around one of the upper edges) to make a square-like shape (a parallelogram) which can be changed to a simple rectangle:



THEN the whole area is bh, which is for both triangles, so just one is ½ × bh.




PDF/Worksheet Links:

  1. Classifying Trianlges
  6. Isosceles and Equilateral Triangles
  7. Interior and Exterior Angles
  8. Triangles and Pythagoras
  10. Triangle Proofs


Video Links:

  1. Area and Perimeter of a Triangle      
  2. Measurement for Similar Triangles
  5. Proof that a Triangle is 180 degrees
  6. Similar Triangles