(696, #37) Prove Property 2 of Theorem 6 for the case n = 2

(696, #37) Prove Property 2 of Theorem 6 for the case n = 2:
If a, b and c are vectors, then a + ( b + c ) = ( a + b ) + c.
Solution:

a + ( b + c) = < a1, a2 > + < b1 + c1, b2 + c2 >
= < a1 + b1 + c1, a2 + b2 + c2 >
= < a1 + b1, a2 + b2 > + < c1, c2>
= ( a + b) + c

by: nl