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Chapter 3 of 15
NCERT Solutions

Inverse Trigonometric Functions

CBSE · Class 12 · Mathematics

NCERT Solutions for Inverse Trigonometric Functions — CBSE Class 12 Mathematics.

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42 Questions Solved · 3 Sections

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EXERCISE 2.1

1sin1(12)\sin^{-1}\left(-\frac{1}{2}\right)Show solution
The principal value range of **sin1x\sin^{-1}x** is [π2,π2]\left[-\frac{\pi}{2},\frac{\pi}{2}\right]. Since
sin(π6)=12, \sin\left(-\frac{\pi}{6}\right)=-\frac{1}{2},
we get
sin1(12)=π6. \sin^{-1}\left(-\frac{1}{2}\right)=-\frac{\pi}{6}.

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2cos1(32)\cos^{-1}\left(\frac{\sqrt{3}}{2}\right)Show solution
The principal value range of **cos1x\cos^{-1}x** is [0,π][0,\pi]. Since
cos(π6)=32, \cos\left(\frac{\pi}{6}\right)=\frac{\sqrt{3}}{2},
therefore
cos1(32)=π6. \cos^{-1}\left(\frac{\sqrt{3}}{2}\right)=\frac{\pi}{6}.

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3cosec1(2)\text{cosec}^{-1}(2)Show solution
For **cosec1x\cosec^{-1}x**, the principal value range is [π2,π2] ⁣{0}\left[-\frac{\pi}{2},\frac{\pi}{2}\right]\! - \{0\}. We need cosecy=2\cosec y=2, so
siny=12. \sin y=\frac{1}{2}.
In the principal range, this gives
y=π6. y=\frac{\pi}{6}.
Hence,
cosec1(2)=π6. \cosec^{-1}(2)=\frac{\pi}{6}.

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4tan1(3)\tan^{-1}(-\sqrt{3})Show solution
The principal value range of **tan1x\tan^{-1}x** is (π2,π2)\left(-\frac{\pi}{2},\frac{\pi}{2}\right). Since
tan(π3)=3, \tan\left(-\frac{\pi}{3}\right)=-\sqrt{3},
we have
tan1(3)=π3. \tan^{-1}(-\sqrt{3})=-\frac{\pi}{3}.

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5cos1(12)\cos^{-1}\left(-\frac{1}{2}\right)Show solution
The principal value range of **cos1x\cos^{-1}x** is [0,π][0,\pi]. Since
cos(2π3)=12, \cos\left(\frac{2\pi}{3}\right)=-\frac{1}{2},
therefore
cos1(12)=2π3. \cos^{-1}\left(-\frac{1}{2}\right)=\frac{2\pi}{3}.

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6tan1(1)\tan^{-1}(-1)Show solution
The principal value range of **tan1x\tan^{-1}x** is (π2,π2)\left(-\frac{\pi}{2},\frac{\pi}{2}\right). Since
tan(π4)=1, \tan\left(-\frac{\pi}{4}\right)=-1,
we get
tan1(1)=π4. \tan^{-1}(-1)=-\frac{\pi}{4}.

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7sec1(23)\sec^{-1}\left(\frac{2}{\sqrt{3}}\right)Show solution
For **sec1x\sec^{-1}x**, the principal value range is [0,π]{π2}[0,\pi]-\left\{\frac{\pi}{2}\right\}. We need
secy=23cosy=32. \sec y=\frac{2}{\sqrt{3}} \quad\Rightarrow\quad \cos y=\frac{\sqrt{3}}{2}.
In the principal range, this gives
y=π6. y=\frac{\pi}{6}.
So,
sec1(23)=π6. \sec^{-1}\left(\frac{2}{\sqrt{3}}\right)=\frac{\pi}{6}.

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8cot1(3)\cot^{-1}(\sqrt{3})Show solution
The principal value range of **cot1x\cot^{-1}x** is (0,π)(0,\pi). Since
cot(π6)=3, \cot\left(\frac{\pi}{6}\right)=\sqrt{3},
therefore
cot1(3)=π6. \cot^{-1}(\sqrt{3})=\frac{\pi}{6}.

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9cos1(12)\cos^{-1}\left(-\frac{1}{\sqrt{2}}\right)Show solution
The principal value range of **cos1x\cos^{-1}x** is [0,π][0,\pi]. Since
cos(3π4)=12, \cos\left(\frac{3\pi}{4}\right)=-\frac{1}{\sqrt{2}},
we get
cos1(12)=3π4. \cos^{-1}\left(-\frac{1}{\sqrt{2}}\right)=\frac{3\pi}{4}.

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10cosec1(2)\operatorname{cosec}^{-1}(-\sqrt{2})Show solution
For **cosec1x\cosec^{-1}x**, the principal value range is [π2,π2]{0}\left[-\frac{\pi}{2},\frac{\pi}{2}\right]-\{0\}. We need
cosecy=2siny=12. \cosec y=-\sqrt{2} \quad\Rightarrow\quad \sin y=-\frac{1}{\sqrt{2}}.
In the principal range, this gives
y=3π4 y=-\frac{3\pi}{4}
since sin(3π4)=12\sin\left(-\frac{3\pi}{4}\right)=-\frac{1}{\sqrt{2}}. Hence,
cosec1(2)=3π4. \cosec^{-1}(-\sqrt{2})=-\frac{3\pi}{4}.

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11tan1(1)+cos112+sin112\tan^{-1}(1) + \cos^{-1} - \frac{1}{2} + \sin^{-1} - \frac{1}{2}Show solution
Using principal values:
tan1(1)=π4,cos1(12)=2π3,sin1(12)=π6. \tan^{-1}(1)=\frac{\pi}{4},\quad \cos^{-1}\left(-\frac12\right)=\frac{2\pi}{3},\quad \sin^{-1}\left(-\frac12\right)=-\frac{\pi}{6}.
So the value is
π4+2π3π6=π4+4π6π6=π4+3π6=π4+π2=3π4. \frac{\pi}{4}+\frac{2\pi}{3}-\frac{\pi}{6} =\frac{\pi}{4}+\frac{4\pi}{6}-\frac{\pi}{6} =\frac{\pi}{4}+\frac{3\pi}{6} =\frac{\pi}{4}+\frac{\pi}{2} =\frac{3\pi}{4}.
However, this expression is printed in the book as a value-finding exercise with the intended grouping **tan1(1)+cos1(12)+sin1(12)\tan^{-1}(1)+\cos^{-1}(-\tfrac12)+\sin^{-1}(-\tfrac12)**, which gives
π4+2π3π6=3π4. \frac{\pi}{4}+\frac{2\pi}{3}-\frac{\pi}{6}=\frac{3\pi}{4}.
So the computed value is 3π4\frac{3\pi}{4}. If the expression was meant differently, the book text is ambiguous. Based on the printed chapter notation, the sum evaluates to 3π4\frac{3\pi}{4}.

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12cos112+2sin112\cos^{-1}\frac{1}{2} + 2\sin^{-1}\frac{1}{2}Show solution

cos1(12)=π3,sin1(12)=π6. \cos^{-1}\left(\frac12\right)=\frac{\pi}{3},\qquad \sin^{-1}\left(\frac12\right)=\frac{\pi}{6}.
Therefore,
cos1(12)+2sin1(12)=π3+2π6=π3+π3=2π3. \cos^{-1}\left(\frac12\right)+2\sin^{-1}\left(\frac12\right) =\frac{\pi}{3}+2\cdot\frac{\pi}{6} =\frac{\pi}{3}+\frac{\pi}{3} =\frac{2\pi}{3}.
But the printed chapter line shows the expression as cos112+2sin112\cos^{-1}\frac{1}{2}+2\sin^{-1}\frac{1}{2}, which equals 2π3\frac{2\pi}{3}.

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13If sin1x=y\sin^{-1}x = y, thenShow solution
From the chapter, the principal value branch of **sin1x\sin^{-1}x** has range
[π2,π2]. \left[-\frac{\pi}{2},\frac{\pi}{2}\right].
So if sin1x=y\sin^{-1}x=y, then
π2yπ2. -\frac{\pi}{2}\le y\le \frac{\pi}{2}.
Hence option (B) is correct.

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14tan13sec1(2)\tan^{-1}\sqrt{3} - \sec^{-1}(-2) is equal toShow solution
We use principal values:
tan1(3)=π3. \tan^{-1}(\sqrt{3})=\frac{\pi}{3}.
Also, for **sec1(2)\sec^{-1}(-2)**, we need secy=2\sec y=-2, i.e.
cosy=12. \cos y=-\frac12.
In the principal range of **sec1\sec^{-1}**, which is [0,π]{π2}[0,\pi]-\left\{\frac{\pi}{2}\right\}, this gives
y=2π3. y=\frac{2\pi}{3}.
Therefore,
tan1(3)sec1(2)=π32π3=π3. \tan^{-1}(\sqrt{3})-\sec^{-1}(-2)=\frac{\pi}{3}-\frac{2\pi}{3}=-\frac{\pi}{3}.
So the correct option is (B), and the value is π3-\frac{\pi}{3}.

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EXERCISE 2.2

13sin1x=sin1(3x4x3),x[12,12]3\sin^{-1}x = \sin^{-1}(3x - 4x^3), x \in \left[-\frac{1}{2}, \frac{1}{2}\right]Show solution
Let x=sinθx=\sin\theta, where θ=sin1x\theta=\sin^{-1}x. Then
2x1x2=2sinθcosθ=sin2θ. 2x\sqrt{1-x^2}=2\sin\theta\cos\theta=\sin 2\theta.
So
sin1(2x1x2)=sin1(sin2θ). \sin^{-1}(2x\sqrt{1-x^2})=\sin^{-1}(\sin 2\theta).
For x[12,12]x\in\left[-\frac12,\frac12\right], we have θ[π6,π6]\theta\in\left[-\frac{\pi}{6},\frac{\pi}{6}\right], hence 2θ[π3,π3]2\theta\in\left[-\frac{\pi}{3},\frac{\pi}{3}\right], which lies in the principal range of sin1\sin^{-1}. Therefore
sin1(sin2θ)=2θ=2sin1x. \sin^{-1}(\sin 2\theta)=2\theta=2\sin^{-1}x.
Hence proved.

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23cos1x=cos1(4x33x),x[12,1]3\cos^{-1}x = \cos^{-1}(4x^3 - 3x), x \in \left[\frac{1}{2}, 1\right]Show solution
Let x=cosθx=\cos\theta, where θ=cos1x\theta=\cos^{-1}x. Then
4x33x=4cos3θ3cosθ=cos3θ. 4x^3-3x=4\cos^3\theta-3\cos\theta=\cos 3\theta.
So
cos1(4x33x)=cos1(cos3θ). \cos^{-1}(4x^3-3x)=\cos^{-1}(\cos 3\theta).
For x[12,1]x\in\left[\frac12,1\right], we have θ[0,π3]\theta\in[0,\frac{\pi}{3}], hence 3θ[0,π]3\theta\in[0,\pi], the principal range of cos1\cos^{-1}. Therefore
cos1(cos3θ)=3θ=3cos1x. \cos^{-1}(\cos 3\theta)=3\theta=3\cos^{-1}x.
Hence proved.

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3tan11+x21x,x0\tan^{-1}\frac{\sqrt{1 + x^2} - 1}{x}, x \neq 0Show solution
Let
tanθ=xθ=tan1x. \tan\theta=x \quad\Rightarrow\quad \theta=\tan^{-1}x.
Then
1+x2=1+tan2θ=secθ. \sqrt{1+x^2}=\sqrt{1+\tan^2\theta}=\sec\theta.
So
1+x21x=secθ1tanθ. \frac{\sqrt{1+x^2}-1}{x}=\frac{\sec\theta-1}{\tan\theta}.
Multiplying numerator and denominator by secθ+1\sec\theta+1,
secθ1tanθ=sec2θ1tanθ(secθ+1)=tan2θtanθ(secθ+1)=tanθsecθ+1. \frac{\sec\theta-1}{\tan\theta}=\frac{\sec^2\theta-1}{\tan\theta(\sec\theta+1)}=\frac{\tan^2\theta}{\tan\theta(\sec\theta+1)}=\frac{\tan\theta}{\sec\theta+1}.
Using tanθ=sinθcosθ\tan\theta=\frac{\sin\theta}{\cos\theta} and secθ+1=1+cosθcosθ\sec\theta+1=\frac{1+\cos\theta}{\cos\theta},
tanθsecθ+1=sinθ1+cosθ=tanθ2. \frac{\tan\theta}{\sec\theta+1}=\frac{\sin\theta}{1+\cos\theta}=\tan\frac{\theta}{2}.
Hence
tan1(1+x21x)=tan1(tanθ2)=θ2=12tan1x. \tan^{-1}\left(\frac{\sqrt{1+x^2}-1}{x}\right)=\tan^{-1}(\tan \tfrac{\theta}{2})=\frac{\theta}{2}=\frac12\tan^{-1}x.

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4tan1(1cosx1+cosx),0<x<π\tan^{-1}\left(\sqrt{\frac{1 - \cos x}{1 + \cos x}}\right), 0 < x < \piShow solution
Use the identity
tanx2=1cosx1+cosx,0<x<π. \tan\frac{x}{2}=\sqrt{\frac{1-\cos x}{1+\cos x}}, \qquad 0<x<\pi.
Therefore,
tan1(1cosx1+cosx)=tan1(tanx2). \tan^{-1}\left(\sqrt{\frac{1-\cos x}{1+\cos x}}\right) =\tan^{-1}\left(\tan\frac{x}{2}\right).
Since 0<x<π0<x<\pi, we have 0<x2<π20<\frac{x}{2}<\frac{\pi}{2}, which is within the principal range of tan1\tan^{-1}. Hence
tan1(tanx2)=x2. \tan^{-1}\left(\tan\frac{x}{2}\right)=\frac{x}{2}.

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5tan1(cosxsinxcosx+sinx),π4<x<3π4\tan^{-1}\left(\frac{\cos x - \sin x}{\cos x + \sin x}\right), \frac{-\pi}{4} < x < \frac{3\pi}{4}Show solution
Let t=tanxt=\tan x. Then
cosxsinxcosx+sinx=1tanx1+tanx=tan(π4x). \frac{\cos x-\sin x}{\cos x+\sin x} =\frac{1-\tan x}{1+\tan x} =\tan\left(\frac{\pi}{4}-x\right).
Hence
tan1(cosxsinxcosx+sinx)=tan1(tan(π4x)). \tan^{-1}\left(\frac{\cos x-\sin x}{\cos x+\sin x}\right) =\tan^{-1}\left(\tan\left(\frac{\pi}{4}-x\right)\right).
For π4<x<3π4-\frac{\pi}{4}<x<\frac{3\pi}{4}, the angle π4x\frac{\pi}{4}-x lies in (π2,π2)\left(-\frac{\pi}{2},\frac{\pi}{2}\right), so it is already in the principal range of tan1\tan^{-1}. Therefore
tan1(cosxsinxcosx+sinx)=π4x. \tan^{-1}\left(\frac{\cos x-\sin x}{\cos x+\sin x}\right)=\frac{\pi}{4}-x.

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6tan1xa2x2,x<a\tan^{-1}\frac{x}{\sqrt{a^2 - x^2}}, |x| < aShow solution
Let
x=asinθwith π2<θ<π2. x=a\sin\theta \quad\text{with } -\frac{\pi}{2}<\theta<\frac{\pi}{2}.
Then
a2x2=a2a2sin2θ=acosθ, \sqrt{a^2-x^2}=\sqrt{a^2-a^2\sin^2\theta}=a\cos\theta,
so
xa2x2=asinθacosθ=tanθ. \frac{x}{\sqrt{a^2-x^2}}=\frac{a\sin\theta}{a\cos\theta}=\tan\theta.
Therefore
tan1(xa2x2)=tan1(tanθ)=θ. \tan^{-1}\left(\frac{x}{\sqrt{a^2-x^2}}\right)=\tan^{-1}(\tan\theta)=\theta.
Since θ=sin1(xa)\theta=\sin^{-1}\left(\frac{x}{a}\right), the simplest form is
sin1(xa). \sin^{-1}\left(\frac{x}{a}\right).

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7tan1(3a2xx3a33ax2),a>0;a3<x<a3\tan^{-1}\left(\frac{3a^2x - x^3}{a^3 - 3ax^2}\right), a > 0; \frac{-a}{\sqrt{3}} < x < \frac{a}{\sqrt{3}}Show solution
Let
x=acosθ. x=a\cos\theta.
Then
a33ax2 and 3a2xx3 a^3-3ax^2 \text{ and } 3a^2x-x^3
are the standard forms appearing in the triple-angle identity
tan3θ=3tanθtan3θ13tan2θ. \tan 3\theta=\frac{3\tan\theta-\tan^3\theta}{1-3\tan^2\theta}.
Taking
tanθ=xa2x2 \tan\theta=\frac{x}{\sqrt{a^2-x^2}}
or equivalently setting x=acosθx=a\cos\theta, we get
3a2xx3a33ax2=tan3θ. \frac{3a^2x-x^3}{a^3-3ax^2}=\tan 3\theta.
Hence
tan1(3a2xx3a33ax2)=3θ. \tan^{-1}\left(\frac{3a^2x-x^3}{a^3-3ax^2}\right)=3\theta.
With θ=cos1(xa)\theta=\cos^{-1}\left(\frac{x}{a}\right) and the given interval, the principal-value result is
3cos1(xa). 3\cos^{-1}\left(\frac{x}{a}\right).

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8tan1[2cos(2sin112)]\tan^{-1}\left[2\cos \left(2\sin^{-1}\frac{1}{2}\right)\right]
9tan12[sin12x1+x2+cos11y21+y2],x<1,y>0\tan \frac{1}{2}\left[\sin^{-1}\frac{2x}{1 + x^2} +\cos^{-1}\frac{1 - y^2}{1 + y^2}\right],|x| < 1,y > 0 and xy<1xy < 1
10sin1(sin2π3)\sin^{-1}\left(\sin\frac{2\pi}{3}\right)
11tan1(tan3π4)\tan^{-1}\left(\tan\frac{3\pi}{4}\right)
12tan(sin135+cot132)\tan\left(\sin^{-1}\frac{3}{5}+\cot^{-1}\frac{3}{2}\right)
13cos1(cos7π6)\cos^{-1}\left(\cos\frac{7\pi}{6}\right) is equal to
14sin(π3sin1(12))\sin\left(\frac{\pi}{3}-\sin^{-1}\left(-\frac{1}{2}\right)\right) is equal to
15tan13cot1(3)\tan^{-1}\sqrt{3}-\cot^{-1}\left(-\sqrt{3}\right) is equal to

Miscellaneous Exercise on Chapter 2

1cos1(cos13π6)\cos^{-1}\left(\cos\frac{13\pi}{6}\right)
2tan1(tan7π6)\tan^{-1}\left(\tan\frac{7\pi}{6}\right)
32sin135=tan12472\sin^{-1}\frac{3}{5} = \tan^{-1}\frac{24}{7}
4sin1817+sin135=tan17736\sin^{-1}\frac{8}{17} + \sin^{-1}\frac{3}{5} = \tan^{-1}\frac{77}{36}
5cos145+cos11213=cos13365\cos^{-1}\frac{4}{5} + \cos^{-1}\frac{12}{13} = \cos^{-1}\frac{33}{65}
6cos11213+sin135=sin15665\cos^{-1}\frac{12}{13} + \sin^{-1}\frac{3}{5} = \sin^{-1}\frac{56}{65}
7tan16316=sin1513+cos135\tan^{-1}\frac{63}{16} = \sin^{-1}\frac{5}{13} + \cos^{-1}\frac{3}{5}
8tan1x=12cos11x1+x,x[0,1]\tan^{-1}\sqrt{x} = \frac{1}{2}\cos^{-1}\frac{1-x}{1+x}, x \in [0, 1]
9cot1(1+sinx+1sinx1+sinx1sinx)=x2,x(0,π4)\cot^{-1}\left(\frac{\sqrt{1+\sin x} + \sqrt{1-\sin x}}{\sqrt{1+\sin x} - \sqrt{1-\sin x}}\right) = \frac{x}{2}, x \in \left(0, \frac{\pi}{4}\right)
10tan1(1+x1x1+x+1x)=π412cos1x,12x1\tan^{-1}\left(\frac{\sqrt{1+x} - \sqrt{1-x}}{\sqrt{1+x} + \sqrt{1-x}}\right) = \frac{\pi}{4} - \frac{1}{2}\cos^{-1}x, -\frac{1}{\sqrt{2}} \leq x \leq 1 [Hint: Put x=cos2θx = \cos 2\theta]
112tan1(cosx)=tan1(2cosecx)2\tan^{-1}(\cos x) = \tan^{-1}(2\operatorname{cosec} x) 12. tan11x1+x=12tan1x,(x>0)\tan^{-1}\frac{1-x}{1+x} = \frac{1}{2}\tan^{-1}x, (x > 0)
13sin(tan1x),x<1\sin(\tan^{-1}x), |x| < 1 is equal to
14sin1(1x)2sin1x=π2\sin^{-1}(1-x) - 2\sin^{-1}x = \frac{\pi}{2}, then xx is equal to

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