a+ib = (x+iy)^2
a = x^2 - y^2
root(a^2+b^2) = x^2+y^2
b = 2xy
find x, find y in terms of a and b. since 2xy = b, only two possible roots.
wlog, take one value of x and one value of y.
compare x+iy with -x+iy.
x + iy = - x + iy
x = 0,
compare
x+iy = x - iy,
y = 0
roots only equal when x = y = 0