vector 2d = vector two dimensional 
-----------------------------------
property: add two vectors ,vec2d   
-----------------------------------
 first vector = vec2d(2,3) 
x coordinate vec2d(x,y) =  2
y coordinate vec2d(x,y) =  3
-----------------------------------
 second vector = vec2d(3,2) 
 result :vec2d(2,3)+vec2d(3,2) = vec2d(5,5) 
x coordinate vec2d(5,5) =  5
y coordinate vec2d(5,5) =  5
-----------------------------------
property: minus two vectors ,vec2d 
-----------------------------------
 result :vec2d(2,3)-vec2d(3,2) = vec2d(-1,1) 
x coordinate vec2d(-1,1) =  -1
y coordinate vec2d(-1,1) =  1