Home > Geometry, Triangles > Pythagoras’ Theorem

Pythagoras’ Theorem

Definition: The square of the hypotenuse is equal to the sum of the squares of the other two sides.


Right angled triangle with squares

What this means is that if we draw three squares where the length of the sides of the squares are equal to the lengths of the sides of the triangle the area of the big square equals the area of the two smaller squares added together.  (note: The hypotenuse is opposite the right angle and this side produces the largest square)

In the diagram above the largest square is the green square so this implies that c^2=a^2+b^2

It follows that if we are finding a shorter side then we could use a^2=c^2-b^2


Find the length of the hypotenuse.


Here a=3\mbox{ and }b=7

So using c^2=a^2+b^2

We get c^2=3^2+7^2=58

\implies c=\sqrt{58}=7.62cm

Find the length of the missing side 


Since we are finding one of the shorter sides we will use a^2=c^2-b^2

Here b=4\mbox{ and }c=12

We get a^2=12^2-4^2=128

\implies a=\sqrt{128}=11.31cm

Find the value of xThis is a right-angled triangle so let’s use c^2=a^2+b^2 


\implies 50^2=x^2+(2x)^2

\implies 2500=5x^2

\implies x^2=500

\implies x=\sqrt{500}=22.36mm

Does 11cm, 59cm, 60cm produce a right-angled triangle?Use c^2=a^2+b^2 where c=60 since that is the longest length. 

\implies 60^2=11^2+59^2

But 3600\neq 3602, therefore this is not a right-angled triangle.


Categories: Geometry, Triangles
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