/sci/ - Science & Math

I was given this thing and know that it's a(sub)1+a(sub)2+a(sub)3+a(sub)4

but the exercise says that the only valid answer is:

a(sub)j = 2 ;
a(sub)j = j ;
a(sub)j = 2 - j ;
a(sub)j = j^2
a(sub)j = 2/j
a(sub)j = sin(j(pi symbol))

How is any of these options correct?

Can somebody explain this basic shit to me?

shouldn't this be a(sub)1+a(sub)2+a(sub)3+a(sub)4 ?

if a(sub)j are just variables how can you sum them?