ML Aggarwal Solution Class 9 Chapter 18 Trigonometric Ratios and Standard Angles Exercise 18.1

 Exercise 18.1

Question 1

Sol :

(i)

7sin30cos60

7(12)(12)

74


(ii)

3sin245+2cos260

3(12)2+2(12)2

32+24

32+12

3+12

42

⇒2


(iii) cos245+sin260+sin230

(12)2+(32)2+(12)2

12+34+14

2+3+14=64=32


(iv) 

cos90+cos245sin30tan45

0+(12)2(12)(1)

0+1212

0+14

0+14=14


Question 2

Sol :

(i)

sin245+cos245tan260

(12)2+(12)2(3)2

12+123

1+123

26

13


(ii) 

sin30sin90+2cos0tan30×tan60

121+2(1)13×3

12+1

1+22

32


(iii) 

43tan230+sin2603cos260+34tan2602tan245

43(13)2+(32)23(12)2+34(3)22(1)2

4313+3434+942

49+942

4×4+9×92×3636=16+811236

2536


Question 3

(i) sin60cos2453tan30+5cos90

3/2(1/2)2313+5(0)

321233+0

333

3333

333

⇒0


(ii)

22cos45cos60+23sin30tan60cos0

221212+231231

⇒1+3-1

⇒3


(iii) 

45tan2602sin23034tan230

45(3)22(12)234(13)2

4532(14)3413

125814

12×48×5×41×55×4

48160520

11720

51720


Question 4

Sol :
(i) 

LHS

cos230+sin30+tan245

(32)2+12+12

34+12+1

3+2+44

94

214


Question 5

Sol :

(i) Given : x=30°

LHS⇒tan 2x

⇒tan2(20°)

⇒tan 60°

⇒√3

RHS2tanx1tan2x

2tan301tan230

2131(13)2

23113

23313

2323

3333

333

⇒√3

∴LHS=RHS


(ii)

Given : x=15°

LHS⇒4sinx.cos4x.sin6x

⇒4sin(2×15).cos(4×15).sin(6×15)

⇒4sin30°×cos 60°×sin 90°

412121

⇒1

∴LHS=RHS


Question 6

Sol :

(i)

1cos2301sin230  (sin2A+cos2A=1)

sin230cos230

(12)2(32)2

1434

13


(ii) 

sin45cos45cos60sin60cos30tan45

12121232321

1232

13


Question 7

Sol :

Given : θ=30°

(i)

LHS⇒sin2θ=sin2.30

=sin 60

=32

RHS⇒2sinθcosθ=2sin30cos30

=21232

=32

∴LHS=RHS


(ii) 

LHS

cos2θ=cos2.30

=cos60

=12

RHS

2cos2θ1=2cos2301

=2(32)21

=2341

=322

=12

∴LHS=RHS

(iii) 

LHS⇒sin3θ

=sin3×30

=sin90

=1


RHS

3sinθ4sin3θ

3sin304sin330

3124(12)3

3248

312

22=1

∴LHS=RHS


(iv)

LHS⇒cos3θ⇒cos3×30°

⇒cos 90°

⇒0


RHS⇒4cos3θ3cosθ

4cos3303cos30

4(32)3332

4338332

332332

⇒0

∴LHS=RHS

Question 8

Sol :

Given : θ=30°

⇒2sinθ : sin2θ

2sinθsin2θ=2sin30sin2×30

=2×12sin60

=132


2sinθsin2θ=23


∴2sinθ : sin2θ=2 : √3


Question 9

Sol :

Given : A=30° ; B=60°

LHS⇒sin(A+B)

⇒sin(30°+60°)

⇒sin 90°

⇒1

RHS⇒sinA+sinB

⇒sin30°+sin60°

12+32

1+32

∴LHS≠RHS


ie., sin(A+B)sinA+sinB.



Question 10

Given : A=60° ; B=30°

Sol :

(i)

LHS⇒sin(A+B)=sin(60+30)

=sin 90=1


RHS⇒sinAcosB+cosAsinB

⇒sin60°cos30°+cos60°sin30°

3232+1212

34+14=3+14=44

=1



(ii)

A=60° ; B=30°

LHS⇒cos(A+B)=cos(60°+30°)

=cos90°=0


RHS⇒cosA.cosB-sinAsinB

⇒cos60°.cos30°-sin60°.sin30°

12323212

3434

⇒0

∴LHS=RHS


i.e. cos(A+B)=cosA.cosB-sinAsinB



(iii) 

LHS

⇒sin(A-B)=sin(60°-30°)

=sin30°

=12


RHS

⇒sinA.cosB-cosAsinB

⇒sin60°.cos30°-cos60°.sin30°

323212.12

3212

312=12=RHS

∴sin(A-B)=sinAcosB-cosAsinB



(iv)

A=60° ; B=30°

LHS⇒tan(A-B)=tan(60°-30°)

=tan 30°

=13



RHS⇒tanAtanB1+tanAtanB

lan60tan301+tan60tan30

3131+313

33132

3132

232

13


∴LHS=RHS

i.e. tan(AB)=tanAtanB1+tanAtanB


Question 11

Sol :

(i) Given : 2sin2θ=√3

sin2θ=32

sin2θ=sin60°

2θ=60°

θ=602

θ=30°



(ii)

Given : 

cos(20+x)=sin60

cos(20+x)=32

cos(20+x)=cos30

20+x=30

x=3020

x=10



(iii)

Given 

3sin2θ=214

3sin2θ=94

sin2θ=94×3

sin2θ=34

sinθ=34

sinθ=32

sinθ=32

sinθ=sin60

θ=60


Question 12

Sol :

Given : 

senθ=cosθ

sinθcosθ=1

tanθ=1

θ=45

2tan2θ+sin2θ1

2tan245+sin2451

2(1)+(12)21

2+121

4+122

32



Question 13

Sol :










(i) From figure 

tanx=31

tanx=3


(ii) x=60

sinθ=32

sinθ=sin60

θ=60



Question 12

Sol :

Given : 

sinθ=cosθ

sinθcosθ=1

tanθ=1

θ=45

2tan2θ+sin2θ1

2tan245+sin2451

2(1)+(12)21

2+121

4+122

32


Question 13

Sol :










(i) From figure


tanx=31

tanx=3


(ii) x=60°



Question 15

Sol :

Given : tan3x=sin45cos45+sin30

tan3x=1212+12

=12+12

=1+12

=22


tan 3x=1


tan 3x=tan 45°

3x=45°

x=15°


Question 16

Sol :

(i) Given : 4cos2x1=0

4cos2x=1

cos2x=14

cos3x=14

cosx=12

cos x=cos 60°

x=60°


(ii) sin2x+cos2xsin260+cos260

(32)2+(12)2

34+14

3+14

44

=1


(iii) cos2xsin2x

cos260sin260

(12)2(32)2

1434

134

24

12


Question 17

Sol :

(i)

Given : secθ=cosecθ

1cosθ=1sinθ

sinθ=cosθ

sinθcosθ=1

tanθ=1

θ=45


(ii) 

Given tanθ=cotθ

tanθcotθ=1 (tanθ=1cotθ)

tan2θ=1

θ=45



Question 18

Sol :

Given : sin3x=1

sin3x=sin90°

3x=90°

x=903

x=30


(i) sinx⇒sin30=12


(ii) cos2x⇒cos 2×30°

⇒cos 60°

12


(iii) tan2xsec2x=tan230sec230

=(13)2(23)2

=1343

=143=33

=-1



Question 19

Sol :

Given : 3tan2θ1=0

3tan2θ=1

tan2θ=13

tanθ=13

θ=30


∴cos2θ=cos2×30°

=cos60°

=12


Question 20

Sol :

Given : sinx+cosy=1

∵x=30°

∴sin30°+cosy=1

12+cosy=1

cosy=112

cosy=12

y=60°


Question 21

Sol :

Given : sin(A+B)=32=cos(A-B)

⇒sin(A+B)=32

⇒sin(A+B)=sin+60°

⇒A+B=60°...(i)

⇒cos(A-B)=32

⇒cos(A-B)=cos30°

⇒A-B=30°...(ii)


From (i) and (ii)


A+B=60AB=302A=90

A=45°


From (i)

⇒A+B=60°

⇒45°+B=60°

⇒B=60°-45°=15°

∴A=45° ; B=15°


Question 22

Sol :












From figure ΔBOC

sin60=BOBC

32=BO8

BO=832

BO=4√3


Similarly from ΔOCD

sin60=ODCD

32=OD8

OD=4√3


∴BD=DO+OD=4√3+4√3=8√3

and from ΔBOC : cos60=OCBC

0C8=12

OC=4


From ΔAOB ; cos60=OAAB

12=OA8

OA=4


∴AC=OA+OC

=4+4=8


Question 23

Sol :









Given : AB=6

∠C=90°

∠B=60°


From Figure 

sin60=ACAB

32=AC6

AC=632

AC=3√3


From Figure 

cos60=BCAB
12=BC6
BC=62
BC=3

Question 24










QC=QB-CB

∵CB=AP=1.8 m

QC=13.8-1.8=12 m


From figure 

tan30=QCPC

13=12PC

PC=12√3

PC=distance of man from building is 12√3



Question 25










ΔABC ; ∠B=45°

∠C=60°
BC=8 m

From figure ΔABD
tan45=ADBD
1=ADBD
AD=BD...(i)

In ΔADC ; Tan60=ADDC
3=ADDC
DC=AD3

∵BC=8
⇒BD+DC=8
AD+AD3=8
(3+1)AD=83
AD=833+1
AD=833+13131
=83(31)31
=8×3832
=24832
=1243

AD=4(33)

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