TRIANGLES

A triangle is one of the basic shapes of geometry: a polygon with three corners or vertices and three sides or edges which are line segments.

The sum of all the internal angles in the triangle is 180º.

How to label a triangle

• The vertices of the triangle are named with capital letters (A, B, C)
• The sides of the triangle are named with the letter of its opposite vertex (a, b, c)
• The angles of the triangle are named with the letter of the vertex and angle symbol (^).

Types of triangles

Triangles are classified two ways:

For their SIDES:

Equilateral
Three equal sides and angles.

Isosceles
Two equal sides and angles.

Scalene
No equal sides or angles.

For their INTERNAL ANGLES

Acute
All their angles are ACUTE.

Right
One RIGHT ANGLE.

In right triangles, the sides that form the right angle are called LEGS and the side that faces them is called HYPOTENUSE.

Obtuse
One OBTUSE angle.

1. EQUILATERAL TRIANGLE (l = 5 cm.)

STEPS:

1. Draw a side.
2. Center your compass in point A and open it to point B, draw an arc.
3. Now center the compass at the point B and open it to point A, draw an arc.
4. Mark the arc intersection point C.
5. Join point A with point C and do the same with point B so you get triangle.

2. ISOSCELES TRIANGLE (a = b = 4,5 cm., c = 3,5 cm.)

STEPS:

1. Draw the different side.
2. Measure the equal side with your compass (4,5 cm.).
3. With this measure draw an arc centering your compass in point A.
4. With the same measure draw another arc centering your compass in point B.
5. Mark the arc intersection point C.
6. Join point A with point C and do the same with point B so you get triangle.

3. SCALENE TRIANGLE (a = 7 cm., b = 4,5 cm., c = 5,5 cm.)

STEPS:

1. Draw a side, for example side a = 7 cm.
2. Measure another side with your compass, for example b = 4,5 cm.
3. With this measure draw an arc centering your compass in point C.
4. Measure the last side with your compass, c = 5,5 cm.
5. With this measure draw an arc centering your compass in point B.
6. Mark the arc intersection point A.
7. Join point A with point C and do the same with point B so you get triangle.

4. RIGHT TRIANGLE (a = 6 (Hypotenuse), b = 3 cm.)

STEPS:

1. Draw the side b = 3 cm.
2. Draw a perpendicular line from point A.
3. Measure side a with your compass, a = 6 cm.
4. With this measure draw an arc centering your compass in point C.
5. Where this arc crosses the perpendicular line to point A we get point B.
6. Join point B with point C so you get triangle.

Practice triangles:

Here you can download a .pdf and then print in a DIN-A4, to practice triangles exercises.
PRACTICE_TRIANGLES