Question

which expression is equivalent to [u+i] [u-i]

Answer 1

the difference of squares .. assuming i is a variable and not an imaginary number

Answer 2

otherwise its the addition of squares :) maybe

Answer 3

I assume it's multiplication:
(U+1)(u-1)=u^2 - (i)^2=u^2 +1

Answer 4

It expands like this:
=(u+i)(u-i)
=u^2 - (u)(i) + (u)(i) - i^2
The non-squared terms are equal and opposite, therefore add to zero.
=u^2 - i^2