Square roots are always nonnegative
Many people get confused, thinking that 
and 
are the same. Though both of these are similar, one of them only has one answer. If we take , we can see that it has 
answers; 
and 
. On the other hand, 
only has one.
Square Root is a function, and a function cannot have two different answers for one input. Technically and are "square roots" of 
, but the square root function only represents one of them--the nonnegative answer.
