quantskillsgmat wrote:can anyone explain me why square root of x is always taken as positive in GMAT...

There is a difference between square root of x and √x. Any number x have two square roots, a negative one and a positive one, i.e. √x and -√x. But √x always represents only the positive root.

For example, 4 have two square roots 2 and - 2. But √4 = 2 not -2.

If the question says square root of 4, then your logic is correct. But I believe the question says √4. Hence, only the positive root has to be considered.

Similarly, any number x has four fourth roots, one positive, one negative, and two imaginary roots. But if the symbol of fourth root is used, then we have to take the positive roots only.