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## Here's the Solution to this Question

A function is invertible if the function is surjective and injective.

Definition. Let $f$  be a function whose domain is a set $X$ . The function $f$  is said to be injective provided that for all $a$  and $b$  in $X$ , whenever $f(a)=f(b)$ , then $a=b$ ; that is, $f(a)=f(b)$  implies $a=b$ . Equivalently, if $a\neq b$ , then $f(a)\neq f(b)$ .

Symbolically,

$\displaystyle \forall a,b\in X,\;\;f(a)=f(b)\Rightarrow a=b$

which is logically equivalent to the contrapositive,

$\displaystyle \forall a,b\in X,\;\;f(a)\neq f(b)\Rightarrow a\neq b$

Definition. A surjective function is a function whose image is equal to its codomain. Equivalently, a function $f$  with domain $X$  and codomain $Y$  is surjective, if for every $y$  in $Y$ , there exists at least one $x$  in $X$  with $\displaystyle f(x)=y$ .

Symbolically,

If $\displaystyle f\colon X\rightarrow Y$ , then $\displaystyle f$  is said to be surjective if

$\displaystyle \forall y\in Y,\,\exists x\in X,\;\;f(x)=y$

Hint: For all these questions, I will consider that $k\equiv\reals$ .

In our case,

(1) $f : \reals\rightarrow\reals^+;\quad f(x)=x^2$

The function $f(x)=x^2$ is not injective in the domain $D(f)=\reals$ , since

$(-3)^2=9=3^2,\quad\text{but}\quad -3\neq3$

Conclusion.

$\boxed{f : \reals\rightarrow\reals^+;\quad f(x)=x^2-\text{no inverse function}}$

(2) $f : \reals^+\rightarrow\reals^+;\quad f(x)=1/x$

This function is surjective and injective, therefore the function has an inverse function.

$y=\frac{1}{x}\longrightarrow x=\frac{1}{y}\longrightarrow\\[0.3cm] \boxed{f(x)=\frac{1}{x}\longrightarrow f^{-1}(x)=\frac{1}{x}}$

Verification,

$f\left(f^{-1}(x)\right)=\frac{1}{\displaystyle\frac{1}{x}}=x$

(3) $f : \reals^+\rightarrow\reals^+;\quad f(x)=x^2$

This function is surjective and injective, therefore the function has an inverse function.

$y=x^2\longrightarrow x=\sqrt{y}\longrightarrow\\[0.3cm] \boxed{f(x)=x^2\longrightarrow f^{-1}(x)=\sqrt{x}}$

Verification,

$f\left(f^{-1}(x)\right)=\left(\sqrt{x}\right)^2=x$

(1) $f : \reals\rightarrow\reals^+;\quad f(x)=x^2$

$f : \reals\rightarrow\reals^+;\quad f(x)=x^2-\text{no inverse function}$

(2) $f : \reals^+\rightarrow\reals^+;\quad f(x)=1/x$

$f(x)=\frac{1}{x}\longrightarrow f^{-1}(x)=\frac{1}{x}$

(3) $f : \reals^+\rightarrow\reals^+;\quad f(x)=x^2$

$f(x)=x^2\longrightarrow f^{-1}(x)=\sqrt{x}$