Chapter 5 Geometry

Section 5.3 All about Triangles

5.3.3 Pythagoras' Theorem

One statement relating the lengths of the sides in a right triangle is provided by Pythagoras' theorem. A commonly-used formulation of the theorem is given here.
Pythagoras' Theorem 5.3.3

Consider a right triangle with the right angle at vertex  C.

The sum of the areas of the squares on the legs a and b equals the area of the square on the hypotenuse c. This statement can be written as an equation (see also the triangle in the figure):

a2 + b2 = c2 .

If the sides of the triangle are denoted in another way, the equation has to be adapted accordingly!

Example 5.3.4
Suppose we have a right triangle with legs (short sides) of length a=6 and b=8.
The length of the hypotenuse can be calculated by means of Pythagoras' theorem:

c= c2 = a2 + b2 =36+64=100=10.

Exercise 5.3.5
Consider a right triangle ABC with the right angle at vertex C, hypotenuse c= 25 3 , and altitude (height) hc =4. The line segment DB has the length q=[ DB ]=3. Here, D is the perpendicular foot of the altitude hc . Calculate the length of the two legs a and b.
Thales' theorem is another important theorem that makes a statement on right triangles.
Thales' Theorem 5.3.6

If the triangle ABC has a right angle at the vertex C, then vertex C lies on a circle with radius r whose diameter 2r is the hypotenuse AB .

The converse statement is also true. Construct a half-circle above a line segment AB . If the points A and B are joint to an arbitrary point C on the half-circle, then the resulting triangle ABC is always right-angled.
Example 5.3.7
Construct a right triangle with a given hypotenuse c=6cm and altitude hc =2.5cm.
  1. First, draw the hypotenuse

    c= AB .

  2. Let the middle of the hypotenuse be the centre of a circle with radius r=c/2.
  3. Then draw a parallel to the hypotenuse at distance hc . This parallel intersects Thales' circle in two points C and C'.
Together with the points A and B, each of these intersections points forms a triangle possessing the required properties, i.e. two solutions exist. Two further solutions are obtained if the construction is repeated drawing a second parallel below the hypotenuse. The constructed triangles are different in position but concerning shape and size these triangles are "congruent" (see also Section 5.3.13).

Exercise 5.3.8
Find the maximum altitude (height) hc of a right triangle with hypotenuse c.