( 25.1, -19.6 ) + ( -5.0, 9.0 ) + ( 12.4, 8.92 ) + ( -20.1, 10.6 )  = 
                  -------------                    ---------------
                                \                /
                                  \            /
                                    \        /
( 25.1, -19.6 ) + ( 12.4, 8.92 ) + ( -25.1, 19.6 ) 
---------------                    ---------------
              \                    /
               \                  /
                \                /
                 \              /
                  \            /
                   ( 0.0, 0.0 )   +  ( 12.4, 8.92 ) = ( 12.4, 8.92 )

Some things which Cannot be Done

In computer science terms, the "+" symbol is overloaded, which means that the operation called for depends on the type of operands. For example:

1.34 + -9.06 
+ means addition of real numbers

( 84.02, 90.31 )T + ( -14.23, 10.85 )T
+ means addition of 2D column matrices

The following have no meaning:

34.5 + ( 84.02, 90.31 )T
can't add a number to a matrix

( 84.02, 90.31 ) + ( -14.23, 10.85 )T
can't add a row matrix to a column matrix

( 84.02, 90.31 ) + ( -14.23, 10.85, 32.75 )
can't add matrices of different dimensions

These problems are clear when the elements are written out, as above, but it is less clear when variable symbols are used:

a + x  <-- can't add a number to a matrix

x + y  <-- make sure that both are of the same type

There are some strange-looking things you can do, such as in the following:

( 4.5, x1, w ) + ( -2.3, 3, y2 )  =