Alright all im on this vector question and im having problems with this question. Part (i) i have no idea what to start with / how i possibly prove the question.

This is the question, help will be deeply appriciated. Thanks, David. Also note that || a || = (a_1 + a_2 + ... + a_n)^1/2

 || a - b || = || a ||\cdot || b || - 2 (a \cdot b).