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.

Please help me prove that if a graph is symmetric with respect to the x-axis and to the y-axis, then it is symmetric with respect to the origin.

share|improve this question
1  
aLok: you seem to want to delete this, but it would be both a pity and a little wasteful of Arturo's effort writing a very nice answer (which is quite well-regarded by the community with iots 12 upvotes and all!) Why do you want to delete this? –  Mariano Suárez-Alvarez Aug 31 '11 at 19:46

1 Answer 1

up vote 16 down vote accepted

A graph is symmetric about the $x$-axis if and only if whenever $(a,b)$ is in the graph, so is $(a,-b)$.

A graph is symmetric about the $y$-axis if and only if whenever $(a,b)$ is in the graph, so is $(-a,b)$.

A graph is symmetric about the origin if and only if whenever $(a,b)$ is in the graph, so is $(-a,-b)$.

Say you have a point $(a,b)$ on the graph. Can you show (say, in a couple of steps), that symmetry about $x$ and symmetry about $y$, together, imply that $(-a,-b)$ has to be in the graph as well?

share|improve this answer
1  
yes.I'am look for a more formal and rigorous proof –  alok Aug 24 '11 at 21:26
1  
"More formal rigorous proof" than the one Arturo gave to you? :-? –  a.r. Aug 24 '11 at 21:33
    
I was wondering if there are other ways of proving it. –  alok Aug 24 '11 at 22:00
2  
@pencil: I described what is, once you fill in the blank left by the final question, a formal and rigorous proof of the fact in question. Yes, there are other ways of proving it, but if your question is "of all the possible ways of proving it, which is the one that I'm (or person X) is thinking of?", then that's not a real question. –  Arturo Magidin Aug 25 '11 at 0:53

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.