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Re: If x^2+y^2=1, is x+y=1 ? [#permalink]
15 Sep 2010, 07:12

1

This post received KUDOS

Expert's post

rraggio wrote:

If x^2+y^2=1, is x+y=1 ?

(1) xy=0 (2) y=0

Thanks!

I think answer B is not correct.

(1) xy=0 --> either x=0 or y=0: if x=0, then x^2+y^2=y^2=1 and y=1 or y=-1, so x+y=0+1=1 (answer YES) or x+y=0-1=-1 (answer NO); if y=0, then x^2+y^2=x^2=1 and x=1 or x=-1, so x+y=1+0=1 (answer YES) or x+y=-1+0=-1 (answer NO); Two different answers. No sufficient.

(2) y=0 --> x^2+y^2=x^2=1 and x=1 or x=-1, so x+y=1+0=1 (answer YES) or x+y=-1+0=-1 (answer NO). Two different answers. No sufficient.

(1)+(2) xy=0 and y=0 --> y=0 and x=1 or x=-1, so x+y=1+0=1 (answer YES) or x+y=-1+0=-1 (answer NO). Two different answers. No sufficient.

Re: If x^2+y^2=1, is x+y=1 ? (1) xy=0 (2) y=0 Thanks [#permalink]
18 Feb 2012, 23:52

Expert's post

pratk wrote:

IMO Answer is D.. Here's why:

x^2+y^2= 1

X^2+y^2 can be written as (x+y)^2-2xy

Therefore (x+y)^2-2xy = 1

From statement 1, xy=0 we get (x+y)^2 = 1 From statement 2, y=0 we get (x+y)^2 = 1

Thus (x+y)=1 -----> Square root of both sides

Both statements are sufficient.

The answer to this question is E, not D.

Consider two sets of numbers, which satisfy stem, as well as both statements and give different values of x+y: If y=0 and x=1 then x+y=1+0=1; If y=0 and x=-1 then x+y=-1+0=-1.

Two different answers. No sufficient.

Answer: E.

Now, the problem in your solution (the red part) is that (x+y)^2=1 means that x+y=1 OR x+y=-1 (you forgot to consider negative root). Basically the same way as x^2=4 means that x=2 or x=-2.

Re: If x^2+y^2=1, is x+y=1 ? (1) xy=0 (2) y=0 Thanks [#permalink]
19 Feb 2012, 07:42

Yes Bunuel, what you mention is correct and I also thought about it that way and this would hold true if ther question would have been phrased differently- perhaps something like : "What is the value of x?" However the question simply asks: is x+y=1? And based on my post above, the answer to that question is Yes using both statements independently.

Not sure if my thinking is correct, guess I have been doing alot of critical reasoning questions so my mind is working in a different way.

Re: If x^2+y^2=1, is x+y=1 ? (1) xy=0 (2) y=0 Thanks [#permalink]
19 Feb 2012, 08:30

Expert's post

pratk wrote:

Yes Bunuel, what you mention is correct and I also thought about it that way and this would hold true if ther question would have been phrased differently- perhaps something like : "What is the value of x?" However the question simply asks: is x+y=1? And based on my post above, the answer to that question is Yes using both statements independently.

Not sure if my thinking is correct, guess I have been doing alot of critical reasoning questions so my mind is working in a different way.

Any thoughts?

No, your thinking is not correct. It's seems that you have some problem with this type of DS question. It's a YES/NO DS question. In a Yes/No Data Sufficiency question, statement(s) is sufficient if the answer is “always yes” or “always no” while a statement(s) is insufficient if the answer is "sometimes yes" and "sometimes no".

Now, we have that even when statements are taken together x+y can equal to 1 as well as -1. So, both statements are not sufficient to give definite YES or definite NO answer to the question whether x+y=1.

Re: If x^2+y^2=1, is x+y=1 ? [#permalink]
27 Jun 2013, 12:20

Bunuel wrote:

rraggio wrote:

If x^2+y^2=1, is x+y=1 ?

(1) xy=0 (2) y=0

Thanks!

I think answer B is not correct.

(1) xy=0 --> either x=0 or y=0: if x=0, then x^2+y^2=y^2=1 and y=1 or y=-1, so x+y=0+1=1 (answer YES) or x+y=0-1=-1 (answer NO); if y=0, then x^2+y^2=x^2=1 and x=1 or x=-1, so x+y=1+0=1 (answer YES) or x+y=-1+0=-1 (answer NO); Two different answers. No sufficient.

(2) y=0 --> x^2+y^2=x^2=1 and x=1 or x=-1, so x+y=1+0=1 (answer YES) or x+y=-1+0=-1 (answer NO). Two different answers. No sufficient.

(1)+(2) xy=0 and y=0 --> y=0 and x=1 or x=-1, so x+y=1+0=1 (answer YES) or x+y=-1+0=-1 (answer NO). Two different answers. No sufficient.

Answer: E.

Hi Bunuel,

In one of posts, I read that "square root function can not give negative result"

So in the solution above, is it ok to assume that Under root Y Square (or X Square) will have 2 values: one positive and one negative.

Re: If x^2+y^2=1, is x+y=1 ? [#permalink]
27 Jun 2013, 12:24

Expert's post

RohanKhera wrote:

Bunuel wrote:

rraggio wrote:

If x^2+y^2=1, is x+y=1 ?

(1) xy=0 (2) y=0

Thanks!

I think answer B is not correct.

(1) xy=0 --> either x=0 or y=0: if x=0, then x^2+y^2=y^2=1 and y=1 or y=-1, so x+y=0+1=1 (answer YES) or x+y=0-1=-1 (answer NO); if y=0, then x^2+y^2=x^2=1 and x=1 or x=-1, so x+y=1+0=1 (answer YES) or x+y=-1+0=-1 (answer NO); Two different answers. No sufficient.

(2) y=0 --> x^2+y^2=x^2=1 and x=1 or x=-1, so x+y=1+0=1 (answer YES) or x+y=-1+0=-1 (answer NO). Two different answers. No sufficient.

(1)+(2) xy=0 and y=0 --> y=0 and x=1 or x=-1, so x+y=1+0=1 (answer YES) or x+y=-1+0=-1 (answer NO). Two different answers. No sufficient.

Answer: E.

Hi Bunuel,

In one of posts, I read that "square root function can not give negative result"

So in the solution above, is it ok to assume that Under root Y Square (or X Square) will have 2 values: one positive and one negative.

Regards

Rohan

I guess you are confused by the part where we have x=1 or x=-1 from x^2=1.

Square root function can not give negative result --> \sqrt{some \ expression}\geq{0}, for example \sqrt{x^2}\geq{0} --> \sqrt{4}=2 (not +2 and -2).

In contrast, the equation x^2=25 has TWO solutions, +5 and -5, because both 5^2 and (-5)^2 equal to 25.

Re: If x^2+y^2=1, is x+y=1 ? [#permalink]
27 Jun 2013, 12:32

So you mean that a square root operation results in 2 solution (positive and negative) only in case of an equation ? And otherwise (in case of non equation) there is only one solution i.e. positive ?

Re: If x^2+y^2=1, is x+y=1 ? [#permalink]
27 Jun 2013, 12:42

Expert's post

RohanKhera wrote:

So you mean that a square root operation results in 2 solution (positive and negative) only in case of an equation ? And otherwise (in case of non equation) there is only one solution i.e. positive ?

Regards,

Rohan

Not sure I understand what you mean. Anyway:

x^2=4 --> x=2 or x=-2.

\sqrt{x}=4 --> x=16. Or x=\sqrt{4} --> x=2. _________________

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