# Math Help - Injective and surjective

1. ## Injective and surjective

Hi everyone,

I have a math problem i am having a hard time solving. Can someone please help me?

Show whether or not the function f:R+ -> R+ defined by f(x) = (1/x^2) is

Injective (one-to-one) or
Surjective(onto)

2. Originally Posted by tironci
Hi everyone,

I have a math problem i am having a hard time solving. Can someone please help me?

Show whether or not the function f:R+ -> R+ defined by f(x) = (1/x^2) is

Injective (one-to-one) or
Surjective(onto)

Say $f(x) = f(y) \implies \frac{1}{x^2} = \frac{1}{y^2} \implies x^2 = y^2 \implies x = y \mbox{ since }x,y>0$.

For any $y\in \mathbb{R}^+$ let $x=1/\sqrt{y}$ then $x\in \mathbb{R}^+$ and $f(x) = y$.

3. ## Injective and surjective

So, it is one-to-one and onto?

Thank you

4. Originally Posted by tironci
So, it is one-to-one and onto?
What is the definition of one-to-one? What is the definition of onto? Does the above post prove those things?