(1) Because there is only one equation 1, which is linear when a=0, there is only one solution A=-1/2.

2. When a≠0, the square of △ = 0 2-4a *1= 0 gives a = 2 A = 1/2 or 3/2.

(2) a≥2 or a≤-2 or a=0 is obtained by solving △≥0, because there is at least one.

(3) Personally, when a≠0, it can be considered as countless quadratic function images, and finally all of them are eliminated, leaving only a=0.

The sum of the elements in A is-1/2, which may be wrong. It's purely personal speculation.