The square of a vector refers to the inner product of the vector and itself.

Namely: (x, y) 2 = | a | 2 = (x, y) (x, y) = x 2+y 2.

A=(x, y) itself is the form of quantity, how to convert it?

The complex number represented by a=(x, y) is x+jy, and its module is | x+jy | = sqrt (x 2+y 2).