Pythagoras’ Theorem

 

The longest side of a right- angled triangle is called the hypotenuse, which  is  always opposite the right-angle.

In any right- angled triangle,the area of the square on the hypotenuse is equal to the sum of the areas of the squares on the other sides.

 

 

For any right-angled triangle, this rule can be used to calculate the length of the hypotenuse if the lengths of the smaller sides are known.

(Hypotenuse)² = (Shortest side)²  + (Other side)²

so
(Longest side)² = (Shortest side)²  + (Other side) ²

To find the length of the hypotenuse

• Sketch triangle
• Mark hypotenuse
• Write out pythagoras' theorem for the triangle
  

   (Hypotenuse)² = (Shortest side) ²  + (Other side) 2

• Solve
• Write out solution

Example

Find the length of the hypotenuse:

 

 

 

 

To find the length of a shorter side

 

• Sketch triangle
• Mark hypotenuse
• Write out pythagoras' theorem for the triangle
   (Hypotenuse)² = (Shortest side)²  + (Other side)²
• Solve
• Write out solution

 

Example

Find the length of the missing side:

 

 

The converse of Pythagoras

 

 If  (Hypotenuse)² = (Shortest side) ²  + (Other side)²
 Then the triangle is right angled.

 

Example

Is this a right angled triangle ?

 

Do not start by writing out Pythagoras' Theorem !

Make separate calculations for hypotenuse and sides.

 

Hidden Pythagoras

Very often, you will need to solve a question where the use of Pythagoras' Theorem does not seem obvious.

 

Example

Calculate the perimeter of triangle ABD. (Give your answer correct to 1 dp.)

 

Finding the perimeter requires the length of CD to be known.

Since ACB is a right angled triangle, Pythagoras' Theorem can be used to find length BC. Triangle BCD is also right angled, so Pythagoras' Theorem can be used again , with the value calculated for BC and the given 11 cm to find CD.

Finally, the lengths can be added to find the perimeter.

so

Thus

Perimeter = 12 + 11 +9 +7.6 = 39.6 cm(1 dp)

 

Examples

The gable of a symmetrical building is painted yellow.

Calculate the area of the painted surface.

 

This is a composite area, so split into two parts:

 

A₁ is a rectangle,

so

A₂ is a triangle,

 

so

To find the perpendicular height, x

Use Pythagoras' Theorem

 

Substituting into the equation for A₂ :

Thus the area of the painted surface

A₁ + A₂ = 50 + 16.6 = 66.6 m²

 

Pythagoras with co-ordinates

Example

Calculate the length of the line that joins the points

A(-5, 10) and B ( 3 ,0 )

Solution :

Plot the points and draw the line.

Complete the right angled triangle

solve using Pythagoras' Theorem

This is the basis for the distance formula