Pythagoras’ Theorem tells us that the sides of a right angled triangle are related in the following way:
If we begin by labelling our triangle with letters and as on the following triangle,
This is a beautifully simple theorem, however it is not obvious. It is named after Pythagoras, not because he discovered it but because he was probably the first to prove it.
Here is a visual, showing a square with side length , a square with side length and a square with side length .
Made by Daniel Pearcy.
Without using any algebra, this illustrates why must be equal to . Again, not obvious to begin with, so keep looking to see why.
The square on the left hand side and the square on the right hand side are presented as having equal area.
The square on the left hand side is made up of four triangles (yellow, blue, red, purple), and one square with area
The square on the right hand side is made up of the same four triangles (the yellow triangle has shorter sides and ), and two squares with areas and .
Because the squares on both left hand side and right hand side include the same four triangles, this leads us to conclude that the remaining areas of the left hand side and right hand side are equal. That is, area of the square length is equal to the sum of the areas of the squares with side lengths and .
That is, .
One difficulty with using letters in any formula is that the letters in a formula and the letters in a problem might not match up. For example, the formula above assumes that we label the hypotenuse , but in any problem the hypotenuse might be called or or or fence or anything else for that matter. That is why it is important to remember that it is the shorter sides squared, then added, that give the hypotenuse squared.
Here are two examples of working this formula:
Example 1 The longest side (opposite the right angle) is not known
Intuitively: To find the longest length, add the two shorter lengths (squared).
Example 2 A shorter side is not known
Intuitively: To find a shorter length, start with the longest length (squared) and take the other shorter length (squared).
(Skill 1) Calculating the third side on a Right Angled Triangle