# Lenght Of Vector

1. Oct 2, 2005

### mind game

what constraints must the elements of three dimensional rotation matrix satisfy in order to preserve length of vector A

2. Oct 2, 2005

### robphy

Here's a start:
$${\vec A'}^\top\vec A'=(R\vec A)^\top(R\vec A)=\vec A^\top\vec A$$
So, what must $$R$$ satisfy? Can you do matrix algebra?