Why √[(-a).(-b)] can't be written as √(-a).√(-b) Is it only because complex number do not work for this statement. Just like here: √ab = √[(-a).(-b)] = √a√bi^2 = -√ab which is wrong. We can separate √(-4)(9) = √-36 = 6i , √4i.√9 =6i, but why can't we separate for two negative numbers inside the root? Is it only because of example in the 2nd point does not satisfies or there is a different reason.