Sign in
Please select an account to continue using cracku.in
↓ →
Let $$f(x)=\log_{e}x$$ and $$g(x)=\frac{x^{4}-2x^{3}+3x^{2}-2x+2}{2x^{2}-2x+1}$$. Then the domain of $$f \circ g$$ is
Let $$f(x) = \ln x$$ and $$g(x) = \frac{x^4 - 2x^3 + 3x^2 - 2x + 2}{2x^2 - 2x + 1}$$. We need to find the domain of $$f \circ g$$.
$$f(g(x)) = \ln(g(x))$$ is defined when $$g(x) > 0$$ and $$g(x)$$ itself is defined (denominator $$\neq 0$$).
$$2x^2 - 2x + 1 = 0$$ has discriminant $$4 - 8 = -4 < 0$$.
Since the leading coefficient is positive and discriminant is negative, $$2x^2 - 2x + 1 > 0$$ for all real $$x$$. So $$g(x)$$ is defined for all $$x \in \mathbb{R}$$.
We perform polynomial division of the numerator by the denominator:
$$x^4 - 2x^3 + 3x^2 - 2x + 2 = (2x^2 - 2x + 1) \cdot q(x) + r(x)$$
Dividing: $$\frac{x^4 - 2x^3 + 3x^2 - 2x + 2}{2x^2 - 2x + 1}$$
$$= \frac{1}{2}x^2 - \frac{1}{2}x + 1 + \frac{-\frac{1}{2}x + 1}{2x^2 - 2x + 1}$$
Alternatively, note that the numerator can be written as:
$$x^4 - 2x^3 + 3x^2 - 2x + 2 = (x^2 - x)^2 + 2(x^2 - x) + 2 = (x^2-x+1)^2 + 1$$
Let us verify: $$(x^2-x+1)^2 = x^4 - 2x^3 + 3x^2 - 2x + 1$$. Adding 1 gives $$x^4 - 2x^3 + 3x^2 - 2x + 2$$. Confirmed.
So the numerator equals $$(x^2 - x + 1)^2 + 1 \geq 1 > 0$$ for all real $$x$$.
Since both numerator and denominator are strictly positive for all $$x \in \mathbb{R}$$, we have $$g(x) > 0$$ for all $$x$$.
The domain of $$f \circ g$$ is all of $$\mathbb{R}$$.
The correct answer is Option 4: $$\mathbb{R}$$.
Create a FREE account and get:
Educational materials for JEE preparation