MartinSC wrote:

I was counting on that, but what about this one:

If (x-1)^2=400, which of the following could be the value of ?

(A) 15

(B) 14

(C) –24

(D) –25

(E) –26

I thought "16" applying the "always positive" mantra, but the explanation goes as follows:

"Work the problem by taking the square root of both sides and solving for x.

Thus,

(x-1)^2=400

x-1= +/-20 (wait, I was not expecting to see the negative root!!)

So x= -19 or x=21 and

x-5=-24 or x-5=16" (which is not even an option).

Seeing that 16 was not among the choices, I could have picked (C)-24, which would have been correct, but it goes against the theory of the positive root...so I was left scratching my head.

Anyone has an idea?

Thks!

When the GMAT provides the square root sign for an even root, such as a square root, fourth root, etc. then the only accepted answer is the positive root. That is:

\(\sqrt{9} = 3\), NOT +3 or -3;

\(\sqrt[4]{16} = 2\), NOT +2 or -2;

Notice that in contrast, the equation \(x^2 = 9\) has TWO solutions, +3 and -3. Because \(x^2 = 9\) means that \(x =-\sqrt{9}=-3\) or \(x=\sqrt{9}=3\).Thanks, but in this case the correct answer is considering the negative root because (C) -24 is the correct answer. If it considered the positive root, the correct answer would have been -16, which is not even among the choices.

question.