Quadratic DS Problem

[Guest]

DS question from Learning Express GMAT preparation

What is the value of a+b?

(1) a^2 + b^2 = 13
(2) 2b = 12/a

The answer given is C but I think the answer should be E. Please confirm.

The reason is that after you combine both statements you get (a+b)^2 = 25, but (a+b) can be 5 or -5. So both statements combine are still insufficient to determine the value of a+b. Is that right?

[unique]

No if a+b = -5 then one of a or b is negative. In that case 2ab will be negative. But 2ab = 12. So I think a+b = 5 and answer is C.

DS question from Learning Express GMAT preparation

What is the value of a+b?

(1) a^2 + b^2 = 13
(2) 2b = 12/a

The answer given is C but I think the answer should be E. Please confirm.

The reason is that after you combine both statements you get (a+b)^2 = 25, but (a+b) can be 5 or -5. So both statements combine are still insufficient to determine the value of a+b. Is that right?

[shaji]

The GMAT Math logic considers only +ve roots, therefore the GMAT answer is C.

Please note that if a=-3 and b=-2 would satisfy a=b=5 and 2ab=12.

Another victim of GMAT Math logic.

No if a+b = -5 then one of a or b is negative. In that case 2ab will be negative. But 2ab = 12. So I think a+b = 5 and answer is C.

DS question from Learning Express GMAT preparation

What is the value of a+b?

(1) a^2 + b^2 = 13
(2) 2b = 12/a

The answer given is C but I think the answer should be E. Please confirm.

The reason is that after you combine both statements you get (a+b)^2 = 25, but (a+b) can be 5 or -5. So both statements combine are still insufficient to determine the value of a+b. Is that right?

[JAMGAJR]

I really do not understand why a+b can not be = -5

In this kind of question, if you find x^2=1, you say that x can not be =-1........

i do not understand

for me....square root of x^2 can not be -x.............only this

[StaceyKoprince]

This is a bad question. The test does not only deal with positive roots; rather, it deals only with real numbers, not imaginary numbers.

From the Official Guide 11th edition, page 114:

"The square root of a negative number is not a real number. Every positive number n has two square roots, one positive and the other negative, but SQRTn denotes the positive number whose square is n. For example, SQRTn denotes 3. The two square roots of 9 are SQRT9 = 3 and -SQRT9 = -3.

You only find the positive root if the test presents you with something under a square root sign. If the test presents you with, say x^2 = 9, then you are expected to find both positive and negative three.

Bizarre, I know, but that's the way the GMAT does it. So this question, as written, is not indicative of the GMAT. Don't study it.