# Inter 1st Year Maths 1A Trigonometric Ratios up to Transformations Solutions Ex 6(d)

Practicing the Intermediate 1st Year Maths 1A Textbook Solutions Inter 1st Year Maths 1A Trigonometric Ratios up to Transformations Solutions Exercise 6(d) will help students to clear their doubts quickly.

## Intermediate 1st Year Maths 1A Trigonometric Ratios up to Transformations Solutions Exercise 6(d)

I.

Question 1.
Simplify
(i) $$\frac{\sin 2 \theta}{1+\cos 2 \theta}$$
Solution:
$$\frac{\sin 2 \theta}{1+\cos 2 \theta}$$
= $$\frac{2 \sin \theta \cos \theta}{2 \cos ^{2} \theta}$$
= tan θ

(ii) $$\frac{3 \cos \theta+\cos 3 \theta}{3 \sin \theta-\sin 3 \theta}$$
Solution:
$$\frac{3 \cos \theta+\cos 3 \theta}{3 \sin \theta-\sin 3 \theta}$$
= $$\frac{3 \cos \theta+4 \cos ^{3} \theta-3 \cos \theta}{3 \sin \theta-\left(3 \sin \theta-4 \sin ^{3} \theta\right)}$$
= $$\frac{4 \cos ^{3} \theta}{4 \sin ^{3} \theta}$$
= cot3θ

Question 2.
Evaluate the following.
(i) 6 sin 20° – 8 sin3 20°
Solution:
6 sin 20° – 8 sin320°
= 2(3 sin 20° – 4 sin320°)
= 2 sin (3 × 20)
= 2 sin 60°
= 2 $$\left[\frac{\sqrt{3}}{2}\right]$$
= √3

(ii) cos272° – sin254°
Solution:

(iii) sin242° – sin212°
Solution:
sin (42 + 12) sin (42 – 12)
= sin 54° . sin 30°
= $$\left[\frac{\sqrt{5}+1}{4}\right] \frac{1}{2}$$
= $$\frac{\sqrt{5}+1}{8}$$

Question 3.
(i) Express $$\frac{\sin 4 \theta}{\sin \theta}$$ interms of cos3θ and cos θ.
Solution:
consider sin 4θ = sin(3θ + θ)
= sin 3θ cos θ + cos 3θ sin θ
= (3 sin θ – 4 sin3θ) cos θ + (4 cos3θ – 3 cos θ) sin θ
= 3 sin θ cos θ – 4 sin3θ cos θ + 4 cos3θ sin θ – 3 cos θ sin θ
= 4 cos3θ sin θ – 4 sin3θ cos θ
= sin θ (4 cos3θ – 4 sin2θ cos θ)
$$\frac{\sin 4 \theta}{\sin \theta}=\frac{\sin \theta\left(4 \cos ^{3} \theta-4 \sin ^{2} \theta \cos \theta\right)}{\sin \theta}$$
= 4 cos3θ – 4(1 – cos2θ) cos θ
= 4 cos3θ – 4 cos θ + 4 cos3θ
= 8 cos3θ – 4 cos θ

(ii) Express cos6A + sin6A in terms of sin 2A.
Solution:
cos6A + sin6A
= (cos2A)3 + (sin2A)3
= (cos2A + sin2A)3 – 3 cos2A sin2A (cos2A + sin2A)
= 1 – 3 cos2A sin2A
= 1 – $$\frac{3}{4}$$ (4 cos2A sin2A)
= 1 – $$\frac{3}{4}$$ (sin22A)

(iii) Express $$\frac{1-\cos \theta+\sin \theta}{1+\cos \theta+\sin \theta}$$ in terms of tan θ/2
Solution:

Question 4.
(i) If sin α = $$\frac{3}{5}$$, where $$\frac{\pi}{2}$$ < α < π, evaluate cos 3α and tan 2α.
Solution:

(ii) If cos A = $$\frac{7}{25}$$ and $$\frac{3 \pi}{2}$$ < A < 2π, then find the value of cot A/2.
Solution:

(iii) If 0 < θ < $$\frac{\pi}{8}$$, show that $$\sqrt{2+\sqrt{2+\sqrt{2+2 \cos 4 \theta)}}}=2 \cos (\theta / 2)$$
Solution:

Question 5.
Find the extreme values of
(i) cos 2x + cos2x
Solution:
cos 2x + cos2x
= 2 cos2x – 1 + cos2x
= 3 cos2x – 1
-1 ≤ cos x ≤ 1
0 ≤ cos2x ≤ 1
0 ≤ 3 cos2x ≤ 3
-1 ≤ 3 cos2x – 1 ≤ 2
maximum value = 2
minimum value = -1

(ii) 3 sin2x + 5 cos2x
Solution:
3 sin2x + 5 cos2x
= 3(1 – cos2x) + 5 cos2x
= 3 – 3 cos2x + 5 cos2x
= 3 + 2 cos2x
-1 ≤ cos x ≤ 1
0 ≤ cos2x ≤ 1
0 ≤ 2 cos2x ≤ 2
3 ≤ 3 + 2 cos2x ≤ 5
maximum value = 5
minimum value = 3

Question 6.
If a ≤ cos θ + 3√2 sin[θ + $$\frac{\pi}{4}$$] + 6 ≤ b, then find the largest value of a and smallest value of b.
Solution:
a ≤ cos θ + 3√2 sin[θ + $$\frac{\pi}{4}$$] + 6 ≤ b
consider cos θ + 3√2 sin[θ + $$\frac{\pi}{4}$$] + 6
= cos θ + 3√2[sin θ cos $$\frac{\pi}{4}$$ + cos θ sin $$\frac{\pi}{4}$$] + 6
= cos θ + 3√2 sin θ $$\frac{1}{\sqrt{2}}$$ + 3√2 cos θ $$\frac{1}{\sqrt{2}}$$ + 6
= cos θ + 3 sin θ + 3 cos θ + 6
= 4 cos θ + 3 sin θ + 6
∴ a = 4, b = 3, c = 6
maximum value = c + $$\sqrt{a^{2}+b^{2}}$$
= 6 + $$\sqrt{16+9}$$
= 6 + 5
= 11
minimum value = c – $$\sqrt{a^{2}+b^{2}}$$
= 6 – 5
= 1

Question 7.
Find the periods for the following functions.
(i) cos4x
Solution:

(ii) $$2 \sin \left[\frac{\pi x}{4}\right]+3 \cos \left[\frac{\pi x}{3}\right]$$
Solution:

(iii) sin2x + 2 cos2x
Solution:
Let f(x) = sin2x + 2 cos2x
= 1 – cos2x + 2 cos2x
= 1 + cos2x
= 1 + $$\frac{1+\cos 2 x}{2}$$
∴ period of cos 2x = $$\frac{2 \pi}{2}$$ = π
∴ period of f(x) = π

(iv) $$2 \sin \left[\frac{\pi}{4}+x\right] \cos x$$
Solution:

(v) $$\frac{5 \sin x+3 \cos x}{4 \sin 2 x+5 \cos x}$$
Solution:
Let f(x) = $$\frac{5 \sin x+3 \cos x}{4 \sin 2 x+5 \cos x}$$
period of sin x = 2π
period of cos x = 2π
period of sin 2x = $$\frac{2 \pi}{2}$$ = π
period of cos x = 2π
L.C.M. of (2π, 2π, π, 2π) = 2π
∴ period of f(x) = 2π

II.

Question 1.
(i) If 0 < A < $$\frac{\pi}{4}$$ and cos A = $$\frac{4}{5}$$, find the values of sin 2A and cos 2A.
Solution:

(ii) For what values of A in the first quadrant, the expression $$\frac{\cot ^{3} A-3 \cot A}{3 \cot ^{2} A-1}$$ is positive?
Solution:

(iii) Prove that $$\frac{\cos 3 A+\sin 3 A}{\cos A-\sin A}=1+2 \sin A$$
Solution:

Question 2.
(i) Prove that $$\cot \left[\frac{\pi}{4}-\theta\right]=\frac{\cos 2 \theta}{1-\sin 2 \theta}$$ and hence find the value of cot 15°.
Solution:

(ii) If θ lies in third quadrant and sin θ = $$\frac{-4}{5}$$, find the values of cosec (θ/2) and tan (θ/2).
Solution:

(iii) If 450° < θ < 540° and sin θ = $$\frac{12}{13}$$, then calculate sin (θ/2) and cos (θ/2)
Solution:

(iv) Prove that $$\frac{1}{\cos 290^{\circ}}+\frac{1}{\sqrt{3} \sin 250^{\circ}}=\frac{4}{\sqrt{3}}$$
Solution:

Question 3.
Prove that
(i) $$\frac{\sin 2 A}{(1-\cos 2 A)} \cdot \frac{(1-\cos A)}{\cos A}=\tan \frac{A}{2}$$
Solution:

(ii) $$\frac{\sin 2 x}{(\sec x+1)} \cdot \frac{\sec 2 x}{(\sec 2 x+1)}$$ = $$\tan \frac{x}{2}$$
Solution:

(iii) $$\frac{\left(\cos ^{3} \theta-\cos 3 \theta\right)}{\cos \theta}+\frac{\left(\sin ^{3} \theta+\sin 3 \theta\right)}{\sin \theta}=\mathbf{3}$$
Solution:

Question 4.
(i) Show that cos A = $$\frac{\cos 3 A}{(2 \cos 2 A-1)}$$. Hence find the value of cos 15°.
Solution:

(ii) Show that sin A = $$\frac{\sin 3 A}{1+2 \cos 2 A}$$. Hence find the value of sin 15°.
Solution:

(iii) Prove that tan α = $$\frac{\sin 2 \alpha}{1+\cos 2 \alpha}$$ and hence deduce the values tan 15° and tan 22$$\frac{1^{\circ}}{2}$$.
Solution:

Question 5.
Prove that
(i) $$\frac{1}{\sin 10^{\circ}}-\frac{\sqrt{3}}{\cos 10^{\circ}}=4$$
Solution:

(ii) √3 cosec 20° – sec 20° = 4
Solution:

(iii) tan 9° – tan 27° – cot 27° + cot 9° = 4
Solution:

(iv) If $$\frac{\sin \alpha}{a}=\frac{\cos \alpha}{b}$$, then prove that a sin 2α + b cos 2α = b
Solution:
Given $$\frac{\sin \alpha}{a}=\frac{\cos \alpha}{b}$$
b sin α = a cos α
L.H.S. = a sin 2α + b cos 2α
= a 2 sin α cos α + b (1 – 2 sin2α)
= 2 sin α (a cos α) + b – 2b sin2α
= 2 sin α (b sin α) + b – 2b sin2α
= 2b sin2α + b – 2b sin2α
= b

Question 6.
(i) In a ∆ABC; if $$\tan \frac{A}{2}=\frac{5}{6}$$ and tan$$\tan \frac{B}{2}=\frac{20}{37}$$, then show that $$\tan \frac{C}{2}=\frac{2}{5}$$
Solution:

(ii) If cos θ = $$\frac{5}{13}$$ and 270° < θ < 360°, evaluate sin (θ/2) and cos (θ/2).
Solution:
cos θ = $$\frac{5}{13}$$
given 270 < θ < 360°
⇒ 135 < $$\frac{\theta}{2}$$ < 180°
∴ θ lies in the fourth quadrant

(iii) If 180° < θ < 270° and sin θ = $$\frac{-4}{5}$$ calculate sin[θ/2] cos[θ/2]
Solution:
Given sin θ = $$\frac{-4}{5}$$
⇒ cos θ = $$\frac{-3}{5}$$
given 180 < θ < 270
∴ θ in the III quadrant
Now 90 < $$\frac{\theta}{2}$$ < 135
∴ $$\frac{\theta}{2}$$ is in II quadrant

Question 7.
(i) Prove that $$\cos ^{2} \frac{\pi}{8}+\cos ^{2} \frac{3 \pi}{8}+\cos ^{2} \frac{5 \pi}{8}+\cos ^{2} \frac{7 \pi}{8}$$ = 2
Solution:

(ii) Show that $$\cos ^{4}\left(\frac{\pi}{8}\right)+\cos ^{4}\left(\frac{3 \pi}{8}\right)+\cos ^{4}\left(\frac{5 \pi}{8}\right)+\cos ^{4}\left(\frac{7 \pi}{8}\right)$$ = $$\frac{3}{2}$$
Solution:

III.

Question 1.
(i) If tan x + tan(x + $$\frac{\pi}{3}$$) + tan(x + $$\frac{2 \pi}{3}$$) = 3, show that tan 3x = 1
Solution:

(ii) Prove that $$\sin \frac{\pi}{5} \sin \frac{2 \pi}{5} \cdot \sin \frac{3 \pi}{5} \cdot \sin \frac{4 \pi}{5}$$ = $$\frac{5}{16}$$
Solution:

(iii) Show that $$\cos ^{2}\left(\frac{\pi}{10}\right)+\cos ^{2}\left(\frac{2 \pi}{5}\right)+\cos ^{2}\left(\frac{3 \pi}{5}\right)$$ + $$\cos ^{2}\left(\frac{9 \pi}{10}\right)$$ = 2
Solution:

Question 2.
(i) $$\frac{1-\sec 8 \alpha}{1-\sec 4 \alpha}=\frac{\tan 8 \alpha}{\tan 2 \alpha}$$
Solution:

(ii) $$\left[1+\cos \frac{\pi}{10}\right]\left[1+\cos \frac{3 \pi}{10}\right]\left[1+\cos \frac{7 \pi}{10}\right]$$ $$\left[1+\cos \frac{9 \pi}{10}\right]=\frac{1}{16}$$
Solution:

Question 3.
(i) Prove that $$\cos \frac{2 \pi}{7} \cdot \cos \frac{4 \pi}{7} \cdot \cos \frac{8 \pi}{7}=\frac{1}{8}$$
Solution:

(ii) $$\cos \frac{\pi}{11} \cdot \cos \frac{2 \pi}{11} \cdot \cos \frac{3 \pi}{11} \cdot \cos \frac{4 \pi}{11}$$ $$\cos \frac{5 \pi}{11}=\frac{1}{32}$$
Solution:

Question 4.
(i) If cos α = $$\frac{3}{5}$$ and cos β = $$\frac{5}{13}$$ and α, β are acute angles, then prove that
(a) $$\sin ^{2}\left[\frac{\alpha-\beta}{2}\right]=\frac{1}{65}$$
(b) $$\cos ^{2}\left[\frac{\alpha+\beta}{2}\right]=\frac{16}{65}$$
Solution:

(ii) If A is not an integral multiple of π, prove that cos A . cos 2A . cos 4A . cos 8A = $$\frac{\sin 16 A}{16 \sin A}$$ and hence deduce that $$\cos \frac{2 \pi}{15} \cdot \cos \frac{4 \pi}{15} \cdot \cos \frac{8 \pi}{15} \cdot \cos \frac{16 \pi}{15}=\frac{1}{16}$$
Solution:
Take 16 sin A {cos A . cos 2A . cos 4A . cos 8A}
= 8(2 sin A . cos A) cos 2A . cos 4A . cos 8A
= 8 sin 2A . cos 2A . cos 4A . cos 8A
= 4(2 sin 2A . cos 2A) . cos 4A . cos 8A
= 4 sin 4A . cos 4A . cos 8A
= 2 (2 sin 4A . cos 4A) . cos 8A
= 2 sin 8A . cos 8A
= sin (16A)
∴ 16 sin A {cos A . cos 2A . cos 4A . cos 8A} = sin (16A)
∴ cos A . cos 2A . cos 4A . cos 8A = $$\frac{\sin (16 A)}{16 \sin A}$$