Equation Qs

[Strongt]

is xy = -1?

1) x=y
2) y=1

Answer is A

and I understand why, but looking at statement 2, I can come up with a value for x

[sam2304]

How can you come up with the value of x using statement 2 ? Or am i missing something here ?

is xy = -1 ?

we cannot substitute y = 1 in xy = -1 and say x = -1. xy can be = 4 or 8 or -12. You can substitute only if it is given as 'if xy = -1'.

A.x = y => is xy = -1 ? No. SUFF.
B.y = 1 => is xy = -1 ? We don't have value of x so INSUFF.

IMO A.

Hope i am right

[Anurag@Gurome]

is xy = -1?

1) x=y
2) y=1

Answer is A

and I understand why, but looking at statement 2, I can come up with a value for x

(1) x = y implies that xy cannot be equal to -1, because for their product to be -1, one of them should be positive and other should be negative, which cannot be the case as x = y. So, statement 1 is sufficient to answer the question asked.

(2)y = 1 implies x * (1) = -1, which implies x = -1. So xy = -1, which is again sufficient to answer the given question.

But there is a contradiction in this question, 1st statement says x = y and the 2nd one says x is not equal to y, so the answer to the main question changes. But this cannot be the case in a good GMAT question.

[Strongt]

How can you come up with the value of x using statement 2 ? Or am i missing something here ?

is xy = -1 ?

we cannot substitute y = 1 in xy = -1 and say x = -1. xy can be = 4 or 8 or -12. You can substitute only if it is given as 'if xy = -1'.

A.x = y => is xy = -1 ? No. SUFF.
B.y = 1 => is xy = -1 ? We don't have value of x so INSUFF.

IMO A.

Hope i am right

You're right

But I thougth we could substitute y in xy = -1. I guess we can't

[Strongt]

is xy = -1?

1) x=y
2) y=1

Answer is A

and I understand why, but looking at statement 2, I can come up with a value for x

(1) x = y implies that xy cannot be equal to -1, because for their product to be -1, one of them should be positive and other should be negative, which cannot be the case as x = y. So, statement 1 is sufficient to answer the question asked.

(2)y = 1 implies x * (1) = -1, which implies x = -1. So xy = -1, which is again sufficient to answer the given question.

But there is a contradiction in this question, 1st statement says x = y and the 2nd one says x is not equal to y, so the answer to the main question changes. But this cannot be the case in a good GMAT question.

I also noticed that!

Question is from Manhattan GMAT (Guide 3)

[Ian Stewart]

is xy = -1?

1) x=y
2) y=1

(2)y = 1 implies x * (1) = -1, which implies x = -1. So xy = -1, which is again sufficient to answer the given question.

You're using the question here as though it were a fact. The question *asks* if xy = -1. We don't know if that's true, so we can't use it to draw any conclusions. Of course, if we assume that xy = -1 is true, then obviously we'll find out that xy = -1 is true - that's the "begging the question" fallacy in logic.

When we look at Statement 2 here, we have exactly one fact that we can use: y = 1. Since we know nothing else, we have no idea if xy = -1 is true. If x = -1, then indeed xy = -1. If x is anything else, then xy will not equal -1. So Statement 2 is not sufficient.

Statement 1 is sufficient, since if x = y, the answer to the question must be 'no'. But if you did try to consider both Statements together, the Statements here are in no way contradictory. Using both Statements, we know that x and y are both equal to 1. There's no contradiction there.