Qus : 3
NIMCET PYQ
3
If
$ \theta = \tan^{-1}\dfrac{1}{1+2} + \tan^{-1}\dfrac{1}{1+2\cdot3} + \tan^{-1}\dfrac{1}{1+3\cdot4} + \ldots + \tan^{-1}\dfrac{1}{1+n(n+1)} $,
then $\tan\theta$ is equal to:
1
$\dfrac{n}{n+1}$ 2
$\dfrac{n+1}{n+2}$ 3
$\dfrac{n}{n+2}$ 4
$\dfrac{n-1}{n+2}$ Go to Discussion
NIMCET Previous Year PYQ
NIMCET NIMCET 2009 PYQ
Solution We use identity
$\tan^{-1}a - \tan^{-1}b = \tan^{-1}\dfrac{a-b}{1+ab}$
Here each term satisfies
$\tan^{-1}\dfrac{1}{1+k(k+1)} = \tan^{-1}(k+1) - \tan^{-1}(k)$
So the series telescopes:
$\theta = \tan^{-1}(n+1) - \tan^{-1}(1)$
Thus
$\tan\theta = \dfrac{(n+1)-1}{1+(n+1)(1)} = \dfrac{n}{n+2}$
Qus : 5
NIMCET PYQ
2
The value of
$\cot^{-1}(21) + \cot^{-1}(13) + \cot^{-1}(-8)$
is:
1
0 2
$\pi$ 3
$\infty$ 4
$\dfrac{\pi}{2}$ Go to Discussion
NIMCET Previous Year PYQ
NIMCET NIMCET 2012 PYQ
Solution Use identity for inverse cotangent:
$\cot^{-1}a + \cot^{-1}b = \cot^{-1}\left(\dfrac{ab - 1}{a + b}\right)$
Apply to the first two terms:
$\cot^{-1}(21) + \cot^{-1}(13)$
$= \cot^{-1}\left(\dfrac{21 \cdot 13 - 1}{21 + 13}\right)$
$= \cot^{-1}\left(\dfrac{273 - 1}{34}\right)$
$= \cot^{-1}\left(\dfrac{272}{34}\right)$
$= \cot^{-1}(8)$
Now the expression becomes:
$\cot^{-1}(8) + \cot^{-1}(-8)$
But we know:
$\cot^{-1}(x) + \cot^{-1}(-x) = \pi$
So,
$\cot^{-1}(8) + \cot^{-1}(-8) = \pi$
$\therefore$ The value is $\pi$.
Qus : 6
NIMCET PYQ
2
If $\tan^{-1}(2x) + \tan^{-1}(3x) = \dfrac{\pi}{4}$, then $x$ is:
1
$\dfrac{1}{6}$ 2
$\dfrac{1}{3}$ 3
$\dfrac{1}{2}$ 4
$\dfrac{1}{4}$ Go to Discussion
NIMCET Previous Year PYQ
NIMCET NIMCET 2009 PYQ
Solution Use formula:
$\tan^{-1}a + \tan^{-1}b = \tan^{-1}\left(\dfrac{a+b}{1-ab}\right)$
So:
$\tan^{-1}\left(\dfrac{2x+3x}{1 - 6x^2}\right) = \dfrac{\pi}{4}$
Thus:
$\dfrac{5x}{1 - 6x^2} = 1$
Solve:
$5x = 1 - 6x^2$
$6x^2 + 5x - 1 = 0$
Quadratic gives roots:
$x = \dfrac{1}{3}$ or $x = -\dfrac{1}{2}$
Qus : 7
NIMCET PYQ
1
Number of solutions for
$\tan^{-1}\sqrt{x(x+1)} + \sin^{-1}\sqrt{x^2 + x + 1} = \dfrac{\pi}{2}$ is:
1
Zero 2
One 3
two 4
infinite Go to Discussion
NIMCET Previous Year PYQ
NIMCET NIMCET 2009 PYQ
Solution Both square-root expressions must be real.
Domain 1:
$x(x+1) \ge 0 \Rightarrow x \le -1 \text{ or } x \ge 0$
Domain 2:
$x^2 + x + 1 > 0$ for all $x$ (always positive)
Equation transforms into identity impossible to satisfy over real domain.
Therefore no real solution.
Qus : 14
NIMCET PYQ
1
The value of A that satisfies the equation asinA + bcosA = c is equal to
1
$\tan^{-1}\left(\dfrac{a}{b}\right) \pm \cos^{-1}\left(\dfrac{c}{\sqrt{a^2 + b^2}}\right)$ 2
$\tan^{-1}\left(\dfrac{c}{b}\right) \pm \cos^{-1}\left(\dfrac{a}{\sqrt{a^2 + b^2}}\right)$ 3
$\tan^{-1}\left(\dfrac{a}{b}\right) \pm \sin^{-1}\left(\dfrac{c}{\sqrt{a^2 + b^2}}\right)$ 4
None of these Go to Discussion
NIMCET Previous Year PYQ
NIMCET NIMCET 2017 PYQ
Solution
Let
$R = \sqrt{a^2 + b^2}$
Choose $\theta$ such that
$\cos \theta = \frac{b}{R}, \quad \sin \theta = \frac{a}{R}$
Then
$a\sin A + b\cos A = R(\sin A \sin \theta + \cos A \cos \theta) = R\cos(A - \theta)$
Given that
$a\sin A + b\cos A = c,$
we get
$R\cos(A - \theta) = c \quad \Rightarrow \quad \cos(A - \theta) = \frac{c}{\sqrt{a^2 + b^2}}$
Hence,
$A - \theta = \pm \cos^{-1}\!\left(\frac{c}{\sqrt{a^2 + b^2}}\right) + 2\pi n, \quad n \in \mathbb{Z}$
and since $\tan \theta = \dfrac{a}{b}$, we have $\theta = \tan^{-1}\!\left(\dfrac{a}{b}\right)$
Therefore,
$\boxed{A = \tan^{-1}\!\left(\frac{a}{b}\right) \pm \cos^{-1}\!\left(\frac{c}{\sqrt{a^2 + b^2}}\right)}$
Qus : 16
NIMCET PYQ
4
If $\sin^{-1}\frac{2a}{1+a^2} - \cos^{-1}\frac{1-b^2}{1+b^2} = \tan^{-1}\frac{2x}{1-x^2}$ then $x$ equals:
1
$a$ 2
$b$ 3
$\dfrac{a+b}{1-ab}$ 4
$\dfrac{a-b}{1+ab}$ Go to Discussion
NIMCET Previous Year PYQ
NIMCET NIMCET 2010 PYQ
Solution $\sin^{-1}\frac{2a}{1+a^2} = 2\tan^{-1} a$
$\cos^{-1}\frac{1-b^2}{1+b^2} = 2\tan^{-1} b$
So equation becomes:
$2\tan^{-1} a - 2\tan^{-1} b = \tan^{-1}\frac{2x}{1-x^2}$
Use identity:
$\tan^{-1}u - \tan^{-1}v = \tan^{-1}\frac{u-v}{1+uv}$
Thus
$2\tan^{-1}\frac{a-b}{1+ab} = \tan^{-1}\frac{2x}{1-x^2}$
This gives:
$x=\frac{a-b}{1+ab}$
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