# a question about proofing surjective

• Oct 15th 2012, 09:00 AM
sethplau
• let f :R{1,-1}-R and f(x)=x/x2-1.
• I am trying to show that the function is surjective,but i have the following questions:
• 1）why we need to divide the case into 2 parts (1):y:R\{0} and (2)y=0 before proving it is surjective.
• 2）and for part(1),why we need to show x:R\{1,-1}. I.e. x isn't equal to +1/-1
• 3）BTW,for the definition of surjective,i don't know the pricinple behind f(x)=y (or why we need to show f(x)=y) (for all y£Y,the exists x£X s.t. y=f(x)

It would be great if you could help me
• Oct 15th 2012, 09:30 AM
Plato
Re: a question about proofing surjective
Quote:

Originally Posted by sethplau
• let f :R{1,-1}-R
• F(x)=x/x2-1.
• I am trying to show that the function is surjective,but i have the following questions:
• 1）why we need to divide the case into 2 parts (1):y:R\{0} and (2)y=0
• 2）and for part(1),why we need to show x:R\{1,-1}. I.e. x isn't equal to +1/-1
• 3）and for the definition of surjective,i don't know the pricinple behind f(x)=y (or why we need to show f(x)=y) (for all y£Y,the exists x£X s.t. y=f(x)

I am not really sure what you want done.
Consider $f: (-1,1)\to\mathbb{R}$ defined be $f(x)=\frac{x}{x^2-1}$.
That is a continuous onto function.
Are you asking to have the onto part proven?
• Oct 15th 2012, 10:24 AM
sethplau
Re: a question about proofing surjective
I want to know how to prove the function is surjective.
Though I've seen the detailed solution which show that the proof should be divied into 2parts
， i don't really know why it should be proved through 2parts~