The fundamental orthogonality theorem

[rayman123]

Homework Statement

Hello everyone! I am trying to do my homework and I wonder if any of you knows what ''the fundamental orthogonality theorem'' is...? (my teacher calls it like that)
I have googled it but the only thing I could find was orthogonality... I could not find something like ''the fundamental orthogonality theorem'' on the internet. Does this exist under a different name?

What exactly is this ''fundamental orthogonality theorem''?

Thanks

Homework Equations

The Attempt at a Solution

 

[lippyka]

The fundamental orthogonality theorem is a theorem related to linear algebra which states that any two non-zero vectors can be written as the sum of two mutually orthogonal vectors. In other words, it states that any two non-zero vectors can be expressed as the sum of two vectors that are perpendicular to each other. This theorem is also sometimes called the Pythagoras Theorem.