Question

Classify the following function $F(x)$ defined in $R \to R$ as injective, surjective, both or none
F(x) = $\dfrac{{{x^2} + x + 1}}{{{x^2} - x + 1}}$
A) Injective but not surjective
B) Surjective but not injective
C) Neither surjective nor injective
D) Both injective and surjective

Answer 1

The function $F(x)$ is said to be injective, that is $F(a) = F(b)$ if and only if $a = b$. So, here we have to examine whether the given function is injective or not by putting values of x at respective points and then equating it. The function $F(x)$ is said to be surjective if for every y in the co-domain there exists x in the domain such that $F(x) = y$.

Complete step-by-step answer:
Firstly, we will check the given function is injective or not:
Let $F(a) = F(b)$ $\forall a,b \in R$
$\Rightarrow \dfrac{{{a^2} + a + 1}}{{{a^2} - a + 1}} = \dfrac{{{b^2} + b + 1}}{{{b^2} - b + 1}}$
Since it cannot be further solved, so, the given function is not injective.
Take $a = 1$ and $b = - 1$ then $F(1) \ne F( - 1)$, then also it is not injective.
Secondly, we will check that given function is surjective or not:
Let $\dfrac{{{x^2} + x + 1}}{{{x^2} - x + 1}} = t$ (say)
On further solving it, we will get the equation:
${x^2} + x + 1 = t({x^2} - x + 1)$
$\Rightarrow (t - 1){x^2} + (t + 1)x + (t - 1) = 0$
Now, finding discriminant of the above, we will get:
$(3t - 1)(3 - t) \geqslant 0$
So, it means $t \in \left[ {\dfrac{1}{3},3} \right]$, which is the range of $F(x)$.
The co-domain and range of F is not the same, hence, the given function is not surjective.

Since, the given function is neither surjective nor injective, hence, the correct answer is option C.

Note: 1) Any function is said to be injective if and only if $F(a) = F(b)$ whenever $a = b$. If this is not the case, the given function is not injective.
2) Any function is said to be surjective if and only if $F(x) = y$ for all x, y belonging to the given domain.
3) Another way to find if a function is surjective or not is that if the co-domain and range of the function is the same, it is surjective.
4) A function is called bijective if it is both one-one and onto. 5) Injective function is also called as one one function.
6) Surjective function is also called as onto function.