Well, mainly cannot understand how can I get the x1x2 + y1y2 <= |z1||z2| from (x1x2 - x2y1)^2 >= 0.

First I found the expression (x1y2 x2y1)^2 >= 0 from (x1x2 + y1y2)^2 <= |z1|2|z2|2 . But I dont know how to get the x1x2 + y1y2 <= |z1||z2| expression.