Inverse Trigonometric Functions
Properties and identities of inverse trig functions
GRB_1000_MCQ
Grade Class 12
Question:
Let $f(x) = \sin^{-1}\left(\dfrac{2x}{1+x^2}\right)$, $g(x) = \cos^{-1}\left(\dfrac{1-x^2}{1+x^2}\right)$ and $h(x) = \tan^{-1}\left(\dfrac{2x}{1-x^2}\right)$, then:
$f(x) + 2\tan^{-1}x = \pi \ \forall \ x \geq 1$
$\dfrac{f(x)}{g(x)} = 1 \ \forall \ x \in [0,1]$
$g(x) + h(x) = \forall \ x \in (-1, 0)$
$\dfrac{\lim_{x \to 1^+}(f(x)+g(x)+h(x))}{\lim_{x \to 1^-}(f(x)+g(x)+h(x))} = 3$
Step-by-Step Solution
Step 1: For $x \geq 1$, the identity $\sin^{-1}\left(\frac{2x}{1+x^2}\right) = \pi - 2\tan^{-1}x$ holds.
Therefore, $f(x) + 2\tan^{-1}x = \pi$.
Step 2: For $x \in [0,1]$, the identities $g(x) = \cos^{-1}\left(\frac{1-x^2}{1+x^2}\right) = 2\tan^{-1}x$ and $f(x) = \sin^{-1}\left(\frac{2x}{1+x^2}\right) = 2\tan^{-1}x$ hold.
Thus, $\frac{f(x)}{g(x)} = 1$.
Step 3: For $x \in (-1,0)$, $g(x) = \cos^{-1}\left(\frac{1-x^2}{1+x^2}\right) = 2\tan^{-1}|x|$. Since $x \in (-1,0)$, $|x| = -x$, so $g(x) = 2\tan^{-1}(-x) = -2\tan^{-1}x$.
Also, for $x \in (-1,0)$, $h(x) = \tan^{-1}\left(\frac{2x}{1-x^2}\right) = 2\tan^{-1}x$.
Therefore, $g(x) + h(x) = (-2\tan^{-1}x) + (2\tan^{-1}x) = 0$.
The statement "$g(x) + h(x) = \forall x \in (-1, 0)$" is syntactically incomplete and does not represent a universally true mathematical identity.
Step 4: Evaluate $\lim_{x \to 1^+}(f(x)+g(x)+h(x))$.
For $x > 1$:
$f(x) = \sin^{-1}\left(\frac{2x}{1+x^2}\right) = \pi - 2\tan^{-1}x$.
$$ \lim_{x \to 1^+} f(x) = \pi - 2\tan^{-1}(1) = \pi - 2\left(\frac{\pi}{4}\right) = \pi - \frac{\pi}{2} = \frac{\pi}{2} $$
$g(x) = \cos^{-1}\left(\frac{1-x^2}{1+x^2}\right) = 2\tan^{-1}x$.
$$ \lim_{x \to 1^+} g(x) = 2\tan^{-1}(1) = 2\left(\frac{\pi}{4}\right) = \frac{\pi}{2} $$
$h(x) = \tan^{-1}\left(\frac{2x}{1-x^2}\right)$. For $x > 1$, $h(x) = 2\tan^{-1}x - \pi$.
$$ \lim_{x \to 1^+} h(x) = 2\tan^{-1}(1) - \pi = 2\left(\frac{\pi}{4}\right) - \pi = \frac{\pi}{2} - \pi = -\frac{\pi}{2} $$
Thus, $\lim_{x \to 1^+}(f(x)+g(x)+h(x)) = \frac{\pi}{2} + \frac{\pi}{2} - \frac{\pi}{2} = \frac{\pi}{2}$.
Step 5: Evaluate $\lim_{x \to 1^-}(f(x)+g(x)+h(x))$.
For $x < 1$ (and $x>0$):
$f(x) = \sin^{-1}\left(\frac{2x}{1+x^2}\right) = 2\tan^{-1}x$.
$$ \lim_{x \to 1^-} f(x) = 2\tan^{-1}(1) = 2\left(\frac{\pi}{4}\right) = \frac{\pi}{2} $$
$g(x) = \cos^{-1}\left(\frac{1-x^2}{1+x^2}\right) = 2\tan^{-1}x$.
$$ \lim_{x \to 1^-} g(x) = 2\tan^{-1}(1) = 2\left(\frac{\pi}{4}\right) = \frac{\pi}{2} $$
$h(x) = \tan^{-1}\left(\frac{2x}{1-x^2}\right) = 2\tan^{-1}x$.
$$ \lim_{x \to 1^-} h(x) = 2\tan^{-1}(1) = 2\left(\frac{\pi}{4}\right) = \frac{\pi}{2} $$
Thus, $\lim_{x \to 1^-}(f(x)+g(x)+h(x)) = \frac{\pi}{2} + \frac{\pi}{2} + \frac{\pi}{2} = \frac{3\pi}{2}$.
Step 6: Compute the ratio of the limits.
$$ \frac{\lim_{x \to 1^+}(f(x)+g(x)+h(x))}{\lim_{x \to 1^-}(f(x)+g(x)+h(x))} = \frac{\frac{\pi}{2}}{\frac{3\pi}{2}} = \frac{1}{3} $$
Correct Answer: 1, 2, 4