Question

let me answer with explanation immediately please

Answer 1

consider a be xi+yj

b be mi+nj

c be ai+bj
now a+(b+c)=(xi+yj)+((m+a)i+(n+b)j)
=> (x+m+a)i+(y+n+b)j

similarly (a+b)+c= ((x+m)i+(y+n)j)+ai+bj
=(x+m+a)i+(y+n+b)j