- #1
Jacobpm64
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Homework Statement
Determine whether each of the following functions is one-to-one, onto, neither or both.
[tex] f : (2, \infty) \rightarrow (1, \infty)[/tex], given by [tex] f(x) = \frac{x}{x-2} [/tex]
The Attempt at a Solution
So, I think this is one-to-one and onto. So i need to prove it.
Claim: If [tex]f : (2, \infty) \rightarrow (1, \infty)[/tex], given by [tex] f(x) = \frac{x}{x-2} [/tex], then f is a one-to-one correspondence.
Proof: Assume [tex]f : (2, \infty) \rightarrow (1, \infty)[/tex], given by [tex] f(x) = \frac{x}{x-2} [/tex].
First we must show that f is one-to-one.
Let [tex] a,b \in (2, \infty)[/tex] such that [tex] f(a) = f(b) [/tex].
Notice that
[tex] f(a) = f(b) [/tex]
[tex] \frac{a}{a-2} = \frac{b}{b-2} [/tex]
[tex] a(b-2) = b(a-2) [/tex]
[tex] ab - 2a = ab - 2b [/tex]
[tex] -2a = -2b [/tex]
[tex] a = b[/tex]
Hence, f is one-to-one. Here, do I need to use the fact that the codomain is (1, infinity) or the domain is (2, infinity)?
Now we need to show that f is onto. Let [tex] b \in (1, \infty) [/tex] and take a = ...
Here I would have solved b = a / (a-2) for a, but I do not know how to solve this. Is there any other way to do this proof besides solving for a then substituting back into show that it gives me f(a) = b?