I assume you are dealing with square roots and radicals. You use the i when the number you are square rooting is negative.There cannot be a negative while square rooting so i is for imaginary number. Don't let that confuse you though. It's basically -1. For example;
√-81
-1 x 81 = -81. Take out the -1. (our i)
√81 = 9
We then put our i after the 9 to get 9i.
Just one more for ya
√-75
-1 x 75 = -75. Take out the -1 (i) again.
√75 = 25 x 3
√25 = 5
5i √3
The i goes on the number outside of the radical.
Hope this helps!
