**Chapter 6 Trigonometry Set 6**

**Chapter 6 Trigonometry Set 6**

**Question 1.Choose the correct alternative answer for the following questions.**

**i. sin θ.cosec θ = ?**

Answer:

Answer:

(A)

**ii. cosec 45° = ?Answer:**

(B)

**iii. 1 + tan ^{2} θ = ?(A) cot^{2} θ(B) cosec^{2} θ(C) sec^{2} θ(D) tan^{2} θAnswer:**

(C)

**iv. When we see at a higher level, from the horizontal line, angle formed is ______(A) angle of elevation.(B) angle of depression.(C) 0(D) straight angle.Answer:**

(A)

**Question 2.If sin θ = 11/61, find the value of cos θ using trigonometric identity.Solution:**
sin θ = … [Given]

We know that,

sin

^{2}θ + cos

^{2}θ = 1

…[Taking square root of both sides]

**Question 3.If tan θ = 2, find the values of other trigonometric ratios.Solution:**

tan θ = 2 …[Given]

We know that,

1 + tan

^{2}θ = sec

^{7}θ

∴ 1 + (2)

^{7}= sec

^{7}θ

∴ 1 + 4 = sec

^{7}θ

∴ sec

^{7}θ = 5

∴ sec θ = √5 …[Taking square root of both sides]

**Question 4.**

We know that,

1 + tan

^{2}θ = sec

^{2}θ

**Question 5.Prove the following:i. sec θ (1 – sin θ) (sec θ + tan θ) = 1ii. (sec θ + tan θ) (1 – sin θ) = cos θiii. sec**

^{2}θ + cosec

^{2}θ = sec

^{2}θ × cosec

^{2}θiv. cot

^{2}θ – tan

^{2}θ = cosec

^{2}θ – sec

^{2}θv. tan

^{4}θ + tan

^{2}θ = sec

^{4}θ – sec

^{2}θvii. sec

^{6}x – tan

^{6}x = 1 + 3 sec

^{2}x × tan

^{2}xProof:

i. L.H.S. = sec θ (1 – sin θ) (sec θ + tan θ)

∴ sec θ (1 – sin θ) (sec θ + tan θ) = 1

ii. L.H.S. = (sec θ + tan θ) (1 – sin θ)

∴ (sec θ + tan θ) (1 – sin θ) = cos θ

iii. L.H.S. = sec^{2} θ + cosec^{2} θ

∴ sec^{2} θ + cosec^{2} θ = sec^{2} θ × cosec^{2} θ

iv. L.H.S. = cot^{2} θ – tan^{2} θ

= (cosec^{2} θ – 1) – (sec^{2} θ – 1)

[∵ tan^{2} θ = sec^{2} θ – 1,

cot^{2} θ = cosec^{2} θ – 1]

= cosec^{2} θ – 1 – sec^{2} θ + 1

cosec^{2} θ – sec^{2} θ

= R.H.S.

∴ cot^{2} θ – tan^{2} θ = cosec^{2} θ – sec^{2} θ

v. L.H.S. = tan^{4} θ + tan^{2} θ

= tan^{2} θ (tan^{2} θ + 1)

= tan^{2} θ. sec^{2} θ

…[∵ 1 + tan^{2} θ = sec^{2} θ]

= (sec^{2} θ – 1) sec^{2} θ

…[∵ tan^{2} θ = sec^{2} θ – 1]

= sec^{4} θ – sec^{2} θ

= R.H.S.

∴ tan^{4} θ + tan^{2} θ = sec^{4} θ – sec^{2} θ

vii. L.H.S. = sec^{6} x – tan^{6} x

= (sec^{2} x)^{3} – tan^{6} x

= (1 + tan^{2} x)^{3} – tan^{6} x …[∵ 1 + tan^{2} θ = sec^{2} θ]

= 1 + 3tan^{2} x + 3(tan^{2} x)^{2} + (tan^{2} x)^{3} – tan^{6} x …[∵ (a + b)^{3} = a^{3} + 3a^{2}b + 3ab^{2} + b^{3}]

= 1 + 3 tan^{2} x (1 + tan^{2} x) + tan^{6} x – tan^{6} x

= 1 + 3 tan^{2} x sec^{2} x …[∵ 1 + tan^{2} θ = sec^{2} θ]

= R.H.S.

∴ sec^{3}x – tan^{6}x = 1 + 3sec^{2}x.tan^{2}x

x. We know that,

sin^{2} θ + cos^{2} θ = 1

∴ 1 – sin^{2} θ = cos^{2} θ

∴ (1 – sin θ) (1 + sin θ) = cos θ. cos θ

**Question 6.A boy standing at a distance of 48 metres from a building observes the top of the building and makes an angle of elevation of 30°. Find the height of the building.Solution:**

Let AB represent the height of the building and point C represent the position of the boy.

Angle of elevation = ∠ACB = 30°

BC = 48 m

In right angled ∆ABC,

tan 30° = ABBC … [By definition]

∴ The height of the building is 16√3 m.

**Question 7.From the top of the lighthouse, an observer looks at a ship and finds the angle of depression to be 30°. If the height of the lighthouse is 100 metres, then find how far the ship is from the lighthouse.Solution:**

Let AB represent the height of lighthouse and point C represent the position of the ship.

Angle of depression ∠PAC 30°

AB = 100m.

Now, ray AP || seg BC

∴ ∠ACB = ∠PAC … [Alternate angles]

∴ ∠ACB = 30°

AB = 100m

**Question 8.Two buildings are in front of each other on a road of width 15 metres. From the top of the first building, having a height of 12 metre, the angle of elevation of the top of the second building is 30°. What is the height of the second building?Solution:**

Let AB and CD represent the heights of the two buildings, and BD represent the width of the road.

Draw seg AM ⊥ seg CD

Angle of elevation = ∠CAM = 30°

AB = 12m

BD = 15m

In ꠸ ABDM,

∠B = ∠D = 90°

∠M 90° …[segAM ⊥ segCD]

∠A 90° …[Remaining angle of ꠸ABDM]

꠸ABDM is a rectangle …[Each angle is 90°]

∴ The height of the second building is 20.65 m.

**Question 9.A ladder on the platform of a fire brigade van can be elevated at an angle of 70° to the maximum. The length of the ladder can be extended upto 20 m. If the platform is 2 m above the ground, find the maximum height from the ground upto which the ladder can reach. (sin 70° = 0.94)Solution:**

Let AB represent the length of the ladder and AE represent the height of the platform.

Draw seg AC ⊥ seg BD.

Angle of elevation = ∠BAC = 70°

AB = 20 m

AE = 2m

In right angled ∆ABC,

∴ BC = 0.94 × 20

= 18.80 m

In ꠸ACDE,

∠E = ∠D = 90°

∠C = 90° … [seg AC ⊥ seg BD]

∴ ∠A = 90° … [Remaining angle of ꠸ACDE]

∴ ꠸ACDE is a rectangle. … [Each angle is 90°]

∴ CD = AE = 2 m … [Opposite sides of a rectangle]

Now, BD = BC + CD … [B – C – D]

= 18.80 + 2

= 20.80 m

∴ The maximum height from the ground upto which the ladder can reach is 20.80 metres.

**Question 10.While landing at an airport, a pilot made an angle of depression of 20°. Average speed of the plane was 200 km/hr. The plane reached the ground after 54 seconds. Find the height at which the plane was when it started landing, (sin 20° = 0.342)Solution:**

Let AC represent the initial height and point A represent the initial position of the plane.

Let point B represent the position where plane lands.

Angle of depression = ∠EAB = 20°

Now, seg AE || seg BC

∴ ∠ABC = ∠EAB … [Alternate angles]

∴ ∠ABC = 20°

∴ AC = 0.342 × 3000

= 1026 m

∴ The plane was at a height of 1026 m when it started landing.