Take the 2-minute tour ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

I am having difficulty understanding how my book came up with this answer.

Define $a \star b =ab+2b$, and suppose $x \star y = y \star x$. Then which of the following must be true?

A. $x+y=1$.

B. $y=0$.

C. $x=y$.

D. $x=-2$.

E. $xy=0$.

How did the text conclude that $x=y$ or C is the answer ?

share|improve this question
    
What does $x \star y$ mean in this context? –  Old John Jul 11 '12 at 20:24
    
What are x,y, anyway? Real numbers? –  DonAntonio Jul 11 '12 at 20:37
    
If $\star$ can be any operation, then the only condition that guarantees $x\star y = y\star x$ is $x=y$. But the question is not which condition guarantees the conclusion $x\star y = y\star x$, but rather, which condition is a necessary conclusion. Unless $\star$ is specified, nothing is a necessary conclusion of the condition. Are you sure they don't specify what $\star$ is, or give conditions that are satisfied by it? –  Arturo Magidin Jul 11 '12 at 20:37
    
There are lots of possible answers depending on what it means. If we don't know which it is, I don't think we can be any help. Surely the text you found it from must explain somewhere?! –  Old John Jul 11 '12 at 20:37
    
Sorry Let me re-edit my post –  Rajeshwar Jul 11 '12 at 20:43

2 Answers 2

up vote 4 down vote accepted

Hint: So you are told that $xy+2y=yx+2x$. Cancel.

share|improve this answer

We need the context to know what operation means $\star$, but i guess the equality $x=y$ must be true only if the operation $\star$ is a kind of operation such like division for example, that doesn't satisfy the conmutative property, and only in case that $x=y$ the equality $x \star y = y \star x$ is satisfied.

Sorry about my English, I hope you can understand me :D

share|improve this answer
    
I think you will have to edit your answer to reflect the edited version of the OP's question. –  M Turgeon Jul 16 '12 at 17:13

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.