Solution of Right Triangles — Test Yourself

The value of angle A is :

1. 45°

2. 30°

3. 60°

4. none of these

Answer

From figure,

AB = BC ....................(1)

By formula,

tan θ = $\dfrac{\text{Perpendicular}}{\text{Base}}$

From figure,

⇒ tan A = $\dfrac{BC}{AB}$

⇒ tan A = $\dfrac{AB}{AB}$ [from equation (1)]

⇒ tan A = 1

⇒ tan A = tan 45°

⇒ A = 45°.

Hence, option 1 is the correct option.

The length of AB is:

1. 20 cm

2. 10 ($\sqrt{3}$ - 1) cm

3. $10\sqrt{3}$ cm

4. $10(\sqrt{3} + 1)$ cm

Answer

Since CD is perpendicular on AB.

By formula,

tan θ = $\dfrac{\text{Perpendicular}}{\text{Base}}$

From figure,

$\Rightarrow \text{tan B} = \dfrac{CD}{DB}\\[1em] \Rightarrow \text{tan 30°} = \dfrac{10}{DB}\\[1em] \Rightarrow \dfrac{1}{\sqrt{3}} = \dfrac{10}{DB}\\[1em] \Rightarrow DB = 10\sqrt{3}.$

As ΔACD is a right angled triangle,

$\Rightarrow \text{tan A} = \dfrac{CD}{AD} \\[1em] \Rightarrow \text{tan 45°} = \dfrac{10}{AD} \\[1em] \Rightarrow 1 = \dfrac{10}{AD} \\[1em] \Rightarrow AD = 10.$

From figure,

AB = AD + DB = 10 + $10\sqrt{3} = 10(1 + \sqrt{3})$ cm.

Hence, option 4 is the correct option.

The length of AB is:

1. (10 - $\sqrt{3}$) cm

2. 10($\sqrt{3}$ - 1) cm

3. 10 $\sqrt{3}$ cm

4. 10($\sqrt{3}$ + 1) cm

Answer

By formula,

tan θ = $\dfrac{\text{Perpendicular}}{\text{Base}}$

In triangle BCD,

$\Rightarrow \text{tan B} = \dfrac{CD}{BC}\\[1em] \Rightarrow \text{tan 45°} = \dfrac{10}{BC}\\[1em] \Rightarrow 1 = \dfrac{10}{BC}\\[1em] \Rightarrow BC = 10.$

In triangle ACD,

$\Rightarrow \text{tan A} = \dfrac{CD}{AC}\\[1em] \Rightarrow \text{tan 30°} = \dfrac{10}{AC}\\[1em] \Rightarrow \dfrac{1}{\sqrt{3}} = \dfrac{10}{AC}\\[1em] \Rightarrow AC = 10\sqrt{3}.$

From figure,

⇒ AB = AC - BC

⇒ AB = $10\sqrt{3}$ - 10 cm

⇒ AB = 10($\sqrt{3}$ - 1) cm.

Hence, option 2 is the correct option.

Length of AB is equal to:

1. $20\sqrt{3}$ cm

2. $\dfrac{20}{\sqrt{3}}$ cm

3. 20 cm

4. none of these

Answer

From figure,

DC = CA = x (let)

By formula,

tan θ = $\dfrac{\text{Perpendicular}}{\text{Base}}$

In triangle ABD,

$\Rightarrow \text{tan D} = \dfrac{AB}{DB} \\[1em] \Rightarrow \text{tan 30°} = \dfrac{AB}{DC + CB} \\[1em] \Rightarrow \dfrac{1}{\sqrt{3}} = \dfrac{AB}{x + 20} \\[1em] \Rightarrow AB = \dfrac{x + 20}{\sqrt{3}}.$

Since ΔABC is a right angled triangle, using pythagoras theorem,

⇒ Hypotenuse² = Base² + Height²

⇒ AC² = CB² + AB²

$\Rightarrow x^2 = 20^2 + \Big(\dfrac{x + 20}{\sqrt{3}}\Big)^2\\[1em] \Rightarrow x^2 = 400 + \Big(\dfrac{x^2 + 400 + 40x}{3}\Big)\\[1em] \Rightarrow x^2 = \Big(\dfrac{1200 + x^2 + 400 + 40x}{3}\Big)\\[1em] \Rightarrow 3x^2 = 1200 + x^2 + 400 + 40x\\[1em] \Rightarrow 3x^2 = 1600 + x^2 + 40x\\[1em] \Rightarrow 3x^2 - 1600 - x^2 - 40x = 0\\[1em] \Rightarrow 2x^2 - 40x - 1600 = 0\\[1em] \Rightarrow 2(x^2 - 20x - 800) = 0 \\[1em] \Rightarrow x^2 - 20x - 800 = 0\\[1em] \Rightarrow x^2 - 40x + 20x - 800 = 0\\[1em] \Rightarrow x(x - 40) + 20(x - 40) = 0\\[1em] \Rightarrow (x - 40)(x + 20) = 0\\[1em] \Rightarrow (x - 40) = 0 \text{ or } (x + 20) = 0\\[1em] \Rightarrow x = 40 \text{ or } x = -20.$

As length of side of triangles cannot be negative. So, x = 40 cm.

DC = AC = x = 40 cm.

$AB = \dfrac{x + 20}{\sqrt{3}} = \dfrac{40 + 20}{\sqrt{3}} = \dfrac{60}{\sqrt{3}} = 20\sqrt{3}$ cm.

Hence, option 1 is the correct option.

Statement 1: At a particular time, the length of the shadow of a 50 m tower is $50\sqrt{3}$ m.

Statement 2: The sun's altitude is 30°.

1. Both the statements are true.

2. Both the statements are false.

3. Statement 1 is true, and statement 2 is false.

4. Statement 1 is false, and statement 2 is true.

Answer

In figure,

Let AB be the tower and BC be the shadow.

Join AC.

Given, AB = 50 m and BC = 50 $\sqrt{3}$ m.

As tower is perpendicular to its shadow, thus ΔABC is a right angled triangle,

By formula,

$\Rightarrow \text{tan θ} = \dfrac{\text{Perpendicular}}{\text{Base}} \\[1em] \Rightarrow \text{tan θ} = \dfrac{50}{50\sqrt{3}} \\[1em] \Rightarrow \text{tan θ} = \dfrac{1}{\sqrt{3}} \\[1em] \Rightarrow \text{tan θ} = \text{tan 30°} \\[1em] \Rightarrow \text{θ} = 30°.$

∴ Both the statements are true.

Hence, option 1 is the correct option.

Statement 1: The distance between B and C = 5 m.

Statement 2: BC = $3\sqrt{3}$ m.

1. Both the statements are true.

2. Both the statements are false.

3. Statement 1 is true, and statement 2 is false.

4. Statement 1 is false, and statement 2 is true.

Answer

By formula,

tan θ = $\dfrac{\text{Perpendicular}}{\text{Base}}$

In ΔABP,

$\Rightarrow \text{tan P} = \dfrac{AB}{BP}\\[1em] \Rightarrow \text{tan 60°} = \dfrac{3}{BP}\\[1em] \Rightarrow \sqrt{3} = \dfrac{3}{BP}\\[1em] \Rightarrow BP = \dfrac{3}{\sqrt{3}}\\[1em] \Rightarrow BP = \sqrt{3}\text{ cm}.$

In ΔPCD,

$\Rightarrow \text{tan P} = \dfrac{DC}{PC}\\[1em] \Rightarrow \text{tan 30°} = \dfrac{2}{PC}\\[1em] \Rightarrow \dfrac{1}{\sqrt{3}} = \dfrac{2}{PC}\\[1em] \Rightarrow PC = 2 \sqrt{3}\text{ cm}.$

From figure,

BC = BP + PC $= \sqrt{3} + 2\sqrt{3} = 3\sqrt{3}$.

∴ Statement 1 is false, and statement 2 is true.

Hence, option 4 is the correct option.

Assertion (A): In rhombus ABCD, angle ABC = 120° and length of its each side is 20 cm. The length of diagonal BD = 20 cm.

Reason (R): In ΔAOB, cos 60° = $\dfrac{\text{OB}}{\text{20 cm}}$ and BD = 2 x OB

1. A is true, but R is false.

2. A is false, but R is true.

3. Both A and R are true, and R is the correct reason for A.

4. Both A and R are true, and R is the incorrect reason for A.

Answer

Given, angle ABC = 120° and length of its each side = 20 cm.

As we know that, each diagonal divides the angles at its endpoints into two equal parts.

The diagonal BD bisects ∠ABC and ∠ADC.

⇒ ∠ABD = ∠CBD = $\dfrac{120°}{2}$ = 60°

By formula,

cos θ = $\dfrac{\text{Base}}{\text{Hypotenuse}}$

We know that,

Diagonals of rhombus are perpendicular to each other.

Thus, triangle AOB is a right angle triangle.

In ΔAOB,

$\Rightarrow \text{cos B} = \dfrac{\text{OB}}{\text{AB}}\\[1em] \Rightarrow \text{cos 60°} = \dfrac{\text{OB}}{20}\\[1em] \Rightarrow \dfrac{1}{2} = \dfrac{\text{OB}}{20}\\[1em] \Rightarrow \text{OB} = \dfrac{20}{2}\\[1em] \Rightarrow \text{OB} = 10$

Since, diagonals of a rhombus bisect each other.

⇒ BD = 2 x OB

So, reason (R) is true.

⇒ BD = 2 x 10 cm = 20 cm

So, assertion (A) is true.

∴ Both A and R are true, and R is the correct reason for A.

Hence, option 3 is the correct option.

Assertion (A): The length of the line AB is 100 m.

Reason (R): tan 45° = $\dfrac{50}{PB}$ ⇒ PB = 50 m and AB = 50 m + 50 m = 100 m

1. A is true, but R is false.

2. A is false, but R is true.

3. Both A and R are true, and R is the correct reason for A.

4. Both A and R are true, and R is the incorrect reason for A.

Answer

From figure,

⇒ BC = AP

⇒ AP = 50 m

By formula,

tan θ = $\dfrac{\text{Perpendicular}}{\text{Base}}$

In ΔPBC,

$\Rightarrow \text{tan 45°} = \dfrac{BC}{PB}\\[1em] \Rightarrow 1 = \dfrac{50}{PB}\\[1em] \Rightarrow PB = 50 \text{ m}.$

From figure,

AB = AP + PB = 50 m + 50 m = 100 m.

∴ Both A and R are true, and R is the correct reason for A.

Hence, option 3 is the correct option.

Find the value of cot x.

Answer

In Δ ABC, according to Pythagoras theorem,

⇒ AC² = BC² + AB² (∵ AC is hypotenuse)

⇒ 2² = BC² + ($\sqrt3$)²

⇒ 4 = BC² + 3

⇒ BC² = 4 - 3

⇒ BC² = 1

⇒ BC = $\sqrt1$

⇒ BC = 1

$\text{cot x} = \dfrac{Base}{Perpendicular}\\[1em] \text{cot x} = \dfrac{CB}{AB}\\[1em] \text{cot x} = \dfrac{1}{\sqrt3}$

Hence, cot x = $\dfrac{1}{\sqrt3}$.

Find the length of AB.

Answer

DE = CB = 30 cm

In Δ ADE,

$\text{tan 45°} = \dfrac{Perpendicular}{Base}\\[1em] ⇒ 1 = \dfrac{AE}{DE}\\[1em] ⇒ 1 = \dfrac{AE}{30}\\[1em] ⇒ AE = 30 cm$

In Δ DEB,

$\text{tan 60°} = \dfrac{Perpendicular}{Base}\\[1em] ⇒ \sqrt3 = \dfrac{EB}{DE}\\[1em] ⇒ \sqrt3 = \dfrac{EB}{30}\\[1em] ⇒ EB = 30\sqrt3 = 51.96$

AB = AE + EB

= 30 + 51.96 cm

= 81.96 cm

Hence, AB = 81.96 cm.

In the given figure, AB and EC are parallel to each other. Sides AD and BC are 2 cm each and are perpendicular to AB.

Given that ∠AED = 60° and ∠ACD = 45°; calculate :

(i) AB

(ii) AC

(iii) AE

Answer

(i) In Δ ADC,

$\text{tan 45°} = \dfrac{Perpendicular}{Base}\\[1em] ⇒ 1 = \dfrac{AD}{DC}\\[1em] ⇒ 1 = \dfrac{2}{DC}\\[1em] ⇒ DC = 2$

DC = AB = 2 cm

Hence, AB = 2 cm.

(ii) In Δ ADC,

$\text{sin 45°} = \dfrac{Perpendicular}{Hypotenuse}\\[1em] ⇒ \dfrac{1}{\sqrt2} = \dfrac{AD}{AC}\\[1em] ⇒ \dfrac{1}{\sqrt2} = \dfrac{2}{AC}\\[1em] ⇒ AC = 2\sqrt2 = 2.83 cm$

Hence, AC = 2.83 cm.

(iii) In Δ ADE,

$\text{sin 60°} = \dfrac{Perpendicular}{Hypotenuse}\\[1em] ⇒ \dfrac{\sqrt3}{2} = \dfrac{AD}{AE}\\[1em] ⇒ \dfrac{\sqrt3}{2} = \dfrac{2}{AE}\\[1em] ⇒ AE = \dfrac{4}{\sqrt3} = 2.31 cm$

Hence, AC = 2.31 cm.

In the given figure, ∠B = 60°, AB = 16 cm and BC = 23 cm. Calculate :

(i) BE

(ii) AC.

Answer

(i) In Δ ABE,

$\text{cos 60°} = \dfrac{Base}{Hypotenuse}\\[1em] ⇒ \dfrac{1}{2} = \dfrac{BE}{AB}\\[1em] ⇒ \dfrac{1}{2} = \dfrac{BE}{16}\\[1em] ⇒ BE = \dfrac{16}{2} = 8 cm$

Hence, BE = 8 cm.

(ii) In Δ ABE, according to Pythagoras theorem,

⇒ AB² = BE² + EA² (∵ AB is hypotenuse)

⇒ 16² = 8² + EA²

⇒ 256 = 64 + EA²

⇒ EA² = 256 - 64

⇒ EA² = 192

⇒ EA = $\sqrt{192}$

⇒ EA = 8 $\sqrt{3}$

EC = BC - BE

= 23 - 8 cm

= 15 cm

In Δ AEC, according to Pythagoras theorem,

⇒ AC² = AE² + EC² (∵ AC is hypotenuse)

⇒ AC² = (8 $\sqrt{3}$)² + 15²

⇒ AC² = 192 + 225

⇒ AC² = 417

⇒ AC = $\sqrt{417}$ = 20.42 cm

Hence, AC = 20.42 cm.

Find :

(i) BC

(ii) AD

(iii) AC

Answer

(i) In Δ ABC,

$\text{tan 30°} = \dfrac{Perpendicular}{Base}\\[1em] ⇒ \dfrac{1}{\sqrt3} = \dfrac{AB}{BC}\\[1em] ⇒ \dfrac{1}{\sqrt3} = \dfrac{12}{BC}\\[1em] ⇒ BC = 12 \sqrt3\\[1em] ⇒ BC = 20.78$

Hence, BC = 20.78 cm.

(ii) In Δ ABC, according to angle sum property,

∠ ABC + ∠ BAC + ∠ ACB = 180°

⇒ 90° + ∠ BAC + 30° = 180°

⇒ 120° + ∠ BAC = 180°

⇒ ∠ BAC = 180° - 120°

⇒ ∠ BAC = 60° = ∠ BAD

In Δ ABD,

$\text{cos 60°} = \dfrac{Base}{Hypotenuse}\\[1em] ⇒ \dfrac{1}{2} = \dfrac{AD}{AB}\\[1em] ⇒ \dfrac{1}{2} = \dfrac{AD}{12}\\[1em] ⇒ AD = \dfrac{12}{2}\\[1em] ⇒ AD = 6$

Hence, AD = 6 cm.

(iii) In Δ ABC,

$\text{sin 30°} = \dfrac{Perpendicular}{Hypotenuse}\\[1em] ⇒ \dfrac{1}{2} = \dfrac{AB}{AC}\\[1em] ⇒ \dfrac{1}{2} = \dfrac{12}{AC}\\[1em] ⇒ AC = 12 \times 2\\[1em] ⇒ AC = 24$

Hence, AC = 24 cm.

In right-angled triangle ABC; ∠B = 90°. Find the magnitude of angle A, if :

(i) AB is ${\sqrt3}$ times of BC.

(ii) BC is ${\sqrt3}$ times of AB.

Answer

(i) AB = ${\sqrt3}$ BC

$\dfrac{AB}{BC} = \sqrt3$

$\text{cot θ} = \dfrac{AB}{BC}\\[1em] \text{cot θ} = \sqrt3\\[1em] \text{cot θ} = \text{cot 30°}\\[1em]$

So, θ = 30°

Hence, magnitude of angle A = 30°.

(ii) BC = ${\sqrt3}$ AB

$\dfrac{BC}{AB} = \sqrt3$

$\text{tan θ} = \dfrac{BC}{AB}\\[1em] \text{tan θ} = \sqrt3\\[1em] \text{tan θ} = \text{tan 60°}\\[1em]$

So, θ = 60°

Hence, magnitude of angle A = 60°.

A ladder is placed against a vertical tower. If the ladder makes an angle of 30° with the ground and reaches upto a height of 15 m of the tower; find the length of the ladder.

Answer

AC is the ladder and BC is the tower.

In Δ ABC,

$\text{sin 30°} = \dfrac{Perpendicular}{Hypotenuse}\\[1em] ⇒ \dfrac{1}{2} = \dfrac{BC}{AC}\\[1em] ⇒ \dfrac{1}{2} = \dfrac{15}{AC}\\[1em] ⇒ AC = 15 \times 2\\[1em] ⇒ AC = 30$

Hence, length of the ladder = 30 m.

A kite is attached to a 100 m long string. Find the greatest height reached by the kite when its string makes an angle of 60° with the level ground.

Answer

AC is the string and BC is the height of kite.

In Δ ABC,

$\text{sin 60°} = \dfrac{Perpendicular}{Hypotenuse}\\[1em] ⇒ \dfrac{\sqrt3}{2} = \dfrac{BC}{AC}\\[1em] ⇒ \dfrac{\sqrt3}{2} = \dfrac{BC}{100}\\[1em] ⇒ BC = \dfrac{\sqrt3 \times 100}{2}\\[1em] ⇒ BC = 50\sqrt3 = 86.60$

Hence, the greatest height reached by the kite = 86.60 m.

Find AB and BC, if :

Answer

Let BC be x cm.

BD = BC + CD = (x + 20) cm

In Δ ABD,

$\text{tan 30°} = \dfrac{Perpendicular}{Base}\\[1em] ⇒ \dfrac{1}{\sqrt3} = \dfrac{AB}{BD}\\[1em] ⇒ \dfrac{1}{\sqrt3} = \dfrac{AB}{x + 20}\\[1em] ⇒ x + 20 = AB\sqrt3 ..............(1)$

In Δ ABC,

$\text{tan 45°} = \dfrac{Perpendicular}{Base}\\[1em] ⇒ 1 = \dfrac{AB}{BC}\\[1em] ⇒ 1 = \dfrac{AB}{x}\\[1em] ⇒ x = AB ..............(2)$

From equation (1),

AB + 20 = AB $\sqrt3$

⇒ AB $\sqrt3$ - AB = 20

⇒ AB($\sqrt3$ - 1) = 20

⇒ AB(1.73 - 1) = 20

⇒ AB x (0.73) = 20

⇒ AB = $\dfrac{20}{0.73}$

⇒ AB = 27.32 cm

And BC = x = AB = 27.32 cm

Hence, AB = BC = 27.32 cm.

Find AB and BC, if :

Answer

Let BC be x cm

BD = BC + CD = x + 20 cm

In Δ ABD,

$\text{tan 30°} = \dfrac{Perpendicular}{Base}\\[1em] ⇒ \dfrac{1}{\sqrt3} = \dfrac{AB}{BD}\\[1em] ⇒ \dfrac{1}{\sqrt3} = \dfrac{AB}{x + 20}\\[1em] ⇒ x + 20 = AB\sqrt3 ..............(1)$

In Δ ABC,

$\text{tan 60°} = \dfrac{Perpendicular}{Base}\\[1em] ⇒ \sqrt3 = \dfrac{AB}{BC}\\[1em] ⇒ \sqrt3 = \dfrac{AB}{x}\\[1em] ⇒ x = \dfrac{AB}{\sqrt3} ..............(2)$

From equation (1),

$\dfrac{AB}{\sqrt3} + 20 = AB\sqrt3\\[1em] ⇒ AB + 20\sqrt3 = 3AB\\[1em] ⇒ 34.64 = 3AB - AB\\[1em] ⇒ AB = \dfrac{34.64}{2}\\[1em] ⇒ AB = 17.32$

From equation (2),

x = $\dfrac{AB}{\sqrt3}$

x = $\dfrac{17.32}{\sqrt3}$

x = 10 cm

Hence, AB = 17.32 cm and BC = 10 cm.

Find AB and BC, if :

Answer

Let BC be x cm

BD = BC + CD = x + 20 cm

In Δ ABD,

$\text{tan 45°} = \dfrac{Perpendicular}{Base}\\[1em] ⇒ 1 = \dfrac{AB}{BD}\\[1em] ⇒ 1 = \dfrac{AB}{x + 20}\\[1em] ⇒ x + 20 = AB ..............(2)$

In Δ ABC,

$\text{tan 60°} = \dfrac{Perpendicular}{Base}\\[1em] ⇒ \sqrt3 = \dfrac{AB}{BC}\\[1em] ⇒ \sqrt3 = \dfrac{AB}{x}\\[1em] ⇒ x = \dfrac{AB}{\sqrt3} ..............(2)$

From equation (1),

$\dfrac{AB}{\sqrt3} + 20 = AB\\[1em] ⇒ AB + 20\sqrt3 = \sqrt3AB\\[1em] ⇒ 34.64 = \sqrt3AB - AB\\[1em] ⇒ 34.64 = AB(1.73 - 1)\\[1em] ⇒ 34.64 = AB \times (0.73)\\[1em] ⇒ AB = \dfrac{34.64}{0.73}\\[1em] ⇒ AB = 47.32$

From equation (2),

x = $\dfrac{AB}{\sqrt3}$

x = $\dfrac{47.32}{\sqrt3}$

x = 27.32 cm

Hence, AB = 47.32 cm and BC = 27.32 cm.

Find PQ, if AB = 150 m, ∠P = 30° and ∠Q = 45°

Answer

In Δ APB,

$\text{tan 30°} = \dfrac{Perpendicular}{Base}\\[1em] ⇒ \dfrac{1}{\sqrt3} = \dfrac{AB}{PB}\\[1em] ⇒ \dfrac{1}{\sqrt3} = \dfrac{150}{PB}\\[1em] ⇒ PB = 150\sqrt3 = 259.80$

And, in Δ ABQ,

$\text{tan 45°} = \dfrac{Perpendicular}{Base}\\[1em] ⇒ 1 = \dfrac{AB}{BQ}\\[1em] ⇒ 1 = \dfrac{150}{BQ}\\[1em] ⇒ BQ = 150$

As, PQ = PB + BQ

= 259.80 + 150 m

= 409.80 m

Hence, PQ = 409.80 m.

Find PQ, if AB = 150 m, ∠P = 30° and ∠Q = 45°

Answer

In Δ APB,

$\text{tan 30°} = \dfrac{Perpendicular}{Base}\\[1em] ⇒ \dfrac{1}{\sqrt3} = \dfrac{AB}{PB}\\[1em] ⇒ \dfrac{1}{\sqrt3} = \dfrac{150}{PB}\\[1em] ⇒ PB = 150\sqrt3 = 259.80$

And, in Δ ABQ,

$\text{tan 45°} = \dfrac{Perpendicular}{Base}\\[1em] ⇒ 1 = \dfrac{AB}{BQ}\\[1em] ⇒ 1 = \dfrac{150}{BQ}\\[1em] ⇒ BQ = 150$

As, PQ = PB - BQ

= 259.80 - 150 m

= 109.80 m

Hence, PQ = 109.80 m.

If $\text{tan x°} = \dfrac{5}{12}$, $\text{tan y°} = \dfrac{3}{4}$ and AB = 48 m; find the length of CD.

Answer

Given:

$\text{tan x°} = \dfrac{5}{12}$, $\text{tan y°} = \dfrac{3}{4}$ and AB = 48 m

Let BC be x m.

AC = AB + BC = (48 + x) m.

In Δ ADC,

$\text{tan x°} = \dfrac{Perpendicular}{Base}\\[1em] ⇒ \dfrac{5}{12} = \dfrac{DC}{AC}\\[1em] ⇒ \dfrac{5}{12} = \dfrac{DC}{48 + x}\\[1em] ⇒ \text{12 CD = 240 + 5x} ................(1)$

And, in Δ BDC,

$\text{tan y°} = \dfrac{Perpendicular}{Base}\\[1em] ⇒ \dfrac{3}{4} = \dfrac{DC}{BC}\\[1em] ⇒ \dfrac{3}{4} = \dfrac{DC}{x}\\[1em] ⇒ 4 CD = 3x \\[1em] ⇒ x = \dfrac{4 CD}{3}................(2)$

From equation (1),

240 + 5 $\times \dfrac{4CD}{3}$ = 12 CD

⇒ 240 + $\dfrac{20CD}{3}$ = 12 CD

⇒ 720 + 20 CD = 36 CD

⇒ 720 = 36 CD - 20 CD

⇒ 16 CD = 720

⇒ CD = $\dfrac{720}{16}$

⇒ CD = 45

Hence, CD = 45 m.

The perimeter of a rhombus is 96 cm and obtuse angle of it is 120°. Find the lengths of its diagonals.

Answer

ABCD is a rhombus.

Perimeter of rhombus = 4 x side

Hence, side AB = BC = CD = DA = $\dfrac{96}{4}$ = 24 cm

∠ ABC = 120°

As we know that diagonal of a rhombus bisect each other at 90°.

In Δ ABO,

∠ ABO = $\dfrac{120°}{2}$ = 60°

$sin 60° = \dfrac{Perpendicular}{Hypotenuse}\\[1em] ⇒ \dfrac{\sqrt3}{2} = \dfrac{AO}{AB}\\[1em] ⇒ \dfrac{\sqrt3}{2} = \dfrac{AO}{24}\\[1em] ⇒ AO = \dfrac{24 \sqrt3}{2}\\[1em] ⇒ AO = 20.78$

∴ AC = 2 x AO = 2 x 20.78 = 41.56 cm

Similarly,

$\text{cos 60°} = \dfrac{Base}{Hypotenuse}\\[1em] ⇒ \dfrac{1}{2} = \dfrac{BO}{AB}\\[1em] ⇒ \dfrac{1}{2} = \dfrac{BO}{24}\\[1em] ⇒ BO = \dfrac{24}{2}\\[1em] ⇒ BO = 12$

∴ BD = 2 x BO = 2 x 12 = 24 cm

Hence, the lengths of the diagonals are: AC = 41.56 cm and BD = 24 cm.