a and b are two position vectors of two distinct points A and B , neither of which is the origin. show that if (modulus) a+b = (modulus) a-b , then a is perpendicular to b using :

a. a vector algebraic method

b. a geometric argument

I don't quite understand what they mean by these^

I've drawn a quick diagram using word, see image below.

http://img14.imageshack.us/img14/1426/vectors.png