KC Sinha Mathematics Solution Class 11 Chapter 7 Trigonometric Functions Exercise 7.1

Exercise 7.1

Page no 7.12

Type 1

Question 1

If $\tan \theta=\frac{a}{b}$, where $0<\theta<\frac{\pi}{4}$ and $b>a>0$, find the values of $\sin 2 \theta, \cos 2 \theta$ and $\tan 2 \theta$.

Question 2

(i) If $\tan \theta=-\frac{3}{4}$ and $\frac{\pi}{2}<\theta<\pi$.

Find the values of $\sin \theta, \cos \theta$ and $\cot \theta$.

Question 3

Prove that $\cot \theta-\tan \theta=2 \cot 2 \theta$

Question 4

$1-\tan ^{2}\left(\frac{\pi}{4}-A\right)$

Prove that $\frac{\left(\frac{\pi}{4}-A\right)}{1+\tan ^{2}\left(\frac{\pi}{4}-A\right)}=\sin 2 A$

Question 5

Prove that $\frac{\cos \theta-\sin \theta}{\cos \theta+\sin \theta}=\sec 2 \theta-\tan 2 \theta$

Question 6

Prove that $\tan \left(\frac{\pi}{4}+\theta\right)-\tan \left(\frac{\pi}{4}-\theta\right)=2 \tan 2 \theta$

Question 7

If $\cos \theta=\frac{1}{2}\left(a+\frac{1}{a}\right)$, show that $\cos 2 \theta=\frac{1}{2}\left(a^{2}+\frac{1}{a^{2}}\right)$

Question 8

 Prove that $\cos ^{2} \theta+\sin ^{2} \theta \cos 2 \beta=\cos ^{2} \beta+\sin ^{2} \beta \cos 2 \theta$

Question 9

Prove that $1+\tan \theta \tan 2 \theta=\sec 2 \theta$

Question 10

Prove that $\frac{1+\sin 2 A-\cos 2 A}{1+\sin 2 A+\cos 2 A}=\tan A$

Question 11

Prove that $\frac{1+\sin 2 \theta}{1-\sin 2 \theta}=\left(\frac{1+\tan \theta}{1-\tan \theta}\right)^{2}$

Question 12

Show that $\frac{1}{\sin 10^{\circ}}-\frac{\sqrt{3}}{\cos 10^{\circ}}=4$

Question 13

Prove that $\operatorname{cosec} A-2 \cot 2 A \cos A=2 \sin A$

Page no 7.13

Question 14

Prove that $\cot ^{2} A-\tan ^{2} A=4 \cot 2 A \operatorname{cosec} 2 A$

Question 15

Prove that $\frac{1+\sin 2 A}{\cos 2 A}=\frac{\cos A+\sin A}{\cos A-\sin A}=\tan \left(\frac{\pi}{4}+A\right)$

Question 16

Prove that $\cos ^{6} A-\sin ^{6} A=\cos 2 A\left(1-\frac{1}{4} \sin ^{2} 2 A\right)$

Question 17

 Prove that $\cos ^{2} \theta+\cos ^{2}\left(\frac{\pi}{3}+\theta\right)+\cos ^{2}\left(\frac{\pi}{3}-\theta\right)=\frac{3}{2}$

Question 18

Prove that

(i) $\frac{2 \cos 2^{n} \theta+1}{2 \cos \theta+1}=(2 \cos \theta-1)(2 \cos 2 \theta-1)\left(2 \cos 2^{2} \theta-1\right) \ldots$ $\left(2 \cos 2^{n-1} \theta-1\right)$

(ii) $\frac{\tan 2^{n} \theta}{\tan \theta}=(1+\sec 2 \theta)\left(1+\sec 2^{2} \theta\right)\left(1+\sec 2^{3} \theta\right) \ldots\left(1+\sec 2^{n} \theta\right)$

Question 19

Prove that $\sin 2 x+2 \sin 4 x+\sin 6 x=4 \cos ^{2} x \sin 4 x$

Question 20

Prove that $\frac{\sin 2^{n} \theta}{\sin \theta}=2^{n} \cos \theta \cos 2 \theta \cos 2^{2} \theta \ldots \cos 2^{n-1} \theta$

Question 21

Show that $3(\sin x-\cos x)^{4}+6(\sin x+\cos x)^{2}+4\left(\sin ^{6} x+\cos ^{6} x\right)=13$

Question 22

Show that $2\left(\sin ^{6} x+\cos ^{6} x\right)-3\left(\sin ^{4} x+\cos ^{4} x\right)+1=0$

Question 23

Show that $\cos ^{2} \theta+\cos ^{2}(\alpha+\theta)-2 \cos \alpha \cos \theta \cos (\alpha+\theta)$ is independent of $\theta$.

Type 2

Question 24

Prove that $4\left(\cos ^{3} 10^{\circ}+\sin ^{3} 20^{\circ}\right)=3\left(\cos 10^{\circ}+\sin 20^{\circ}\right)$

Question 25

Prove that $\sin \theta \cos ^{3} \theta-\cos \theta \sin ^{3} \theta=\frac{1}{4} \sin 4 \theta$

Question 26

Prove that $\cos ^{3} \theta \cdot \sin 3 \theta+\sin ^{3} \theta \cdot \cos 3 \theta=\frac{3}{4} \sin 4 \theta$

Question 28

Prove that $4 \sin \theta \sin \left(\theta+\frac{\pi}{3}\right) \sin \left(\theta+\frac{2 \pi}{3}\right)=\sin 3 \theta$

Question 29

Prove that $\cot \theta+\cot \left(60^{\circ}+\theta\right)+\cot \left(120^{\circ}+\theta\right)=3 \cot 3 \theta$

Type 3

Question 30

Prove that $\cos 4 x=1-8 \sin ^{2} x \cos ^{2} x$

Question 31

Prove that $\sin 5 \theta=5 \sin \theta-20 \sin ^{3} \theta+16 \sin ^{5} \theta$

Question 32

 Prove that, $\cos 6 \theta=32 \cos ^{6} \theta-48 \cos ^{4} \theta+18 \cos ^{4} \theta-1$

Question 33

Prove that

$\cos 4 \theta-\cos 4 \alpha=8(\cos \theta-\cos \alpha)(\cos \theta+\cos \alpha)(\cos \theta-\sin \alpha)(\cos\theta+\sin \alpha)$

Question 34

Prove that $\tan 4 \theta=\frac{4 \tan \theta-4 \tan ^{3} \theta}{1-6 \tan ^{2} \theta+\tan ^{4} \theta}$

Page no 7.14

Question 35

If $\tan x=\frac{b}{a}$, prove that $a \cos 2 x+b \sin 2 x=a$

Question 36

If $\tan ^{2} \theta=1+2 \tan ^{2} \phi$, prove that $\cos 2 \phi=1+2 \cos 2 \theta$

Question 37

If $\alpha$ and $\beta$ are acute angles and $\cos 2 \alpha=\frac{3 \cos 2 \beta-1}{3-\cos 2 \beta}$

Prove that $\tan \alpha=\sqrt{2} \tan \beta$

Question 38

If $\tan \beta=3 \tan \alpha$, prove that $\tan (\alpha+\beta)=\frac{2 \sin 2 \beta}{1+2 \cos 2 \beta}$

Question 39

If $x \sin \alpha=y \cos \alpha$, prove that $\frac{x}{\sec 2 \alpha}+\frac{y}{\operatorname{cosec} 2 \beta}=x$

Question 40

If $\tan \theta=\sec 2 \alpha$, prove that $\sin 2 \theta=\frac{1-\tan ^{4} \alpha}{1+\tan ^{4} \alpha}$

Question 41

If $\alpha=\frac{\pi}{3} \cdot$ prove that $\cos \alpha \cdot \cos 2 \alpha \cdot \cos 3 \alpha \cdot \cos 4 \alpha \cdot \cos 5 \alpha \cos 6 \alpha=\frac{1}{16}$

Question 42

If $\alpha=\frac{\pi}{15}$, prove that $\cos 2 \alpha \cos 4 \alpha \cos 8 \alpha \cos 14 \alpha=\frac{1}{16}$

Question 43

If $\tan A \tan B=\sqrt{\frac{a-b}{a+b}}$, prove that

$(a-b \cos 2 A)(a-b \cos 2 B)=a^{2}-b^{2}$