a+b = 2e1-2e2+4e1+e2
= 6e1 - e2
其中e1=(1,0),e2=(0,1).
a +b =(6,0) - (0,1)
= (6,-1)
a+b的模|a+b| = (6^2+1^2)^(1/2) = 根号37