thanks a lot, the information was so useful and it really gave me Idea about SF's but one question remains, how do we find all the roots of polynomial? before when working with reals, we would say whenever polynomial meets the x axis, which is the solution to the x is zero , then we have a root, but now complex variables are in our way. ie how did you find:

all I could do was to say (x^2+i)(x^2-i)= (x^2+i)(x-root(i))(x+root (i))

which then I dont know what root(i) is!

[IMG]file:///C:/Users/HAMID/AppData/Local/Temp/moz-screenshot.png[/IMG][IMG]file:///C:/Users/HAMID/AppData/Local/Temp/moz-screenshot-1.png[/IMG]