Orthogonal Transformations

I am reading Grove and Benson's book on Finite Reflection Groups and am struggling with some of the basic linear algebra.

Some terminology from Grove and Benson:

V is a real Euclidean vector space

A transformation of V is understood to be a linear transformation

The group of all orthogonal transformations of V will be denoted O(V)

Then in chapter 2, Grove and Benson write the following:

If T [TEX]\inEX] O(V), then T is completely determined by its action on the basis vectors = (1,0) and = (0,1).

**If T , then and **

Can someone please help by proving why the last statement is true?