ML Aggarwal Solution Class 9 Chapter 18 Trigonometric Ratios and Standard Angles Exercise 18.1
Exercise 18.1
Question 1
Sol :
(i)
7sin30∘cos60∘
⇒7(12)(12)
⇒74
(ii)
3sin245∘+2cos260∘
⇒3⋅(1√2)2+2⋅(12)2
⇒32+24
⇒32+12
⇒3+12
⇒42
⇒2
(iii) cos245∘+sin260∘+sin230∘
⇒(1√2)2+(√32)2+(12)2
⇒12+34+14
⇒2+3+14=64=32
(iv)
cos90∘+cos245∘sin30∘tan45∘
⇒0+(1√2)2⋅(12)⋅(1)
⇒0+12⋅12
⇒0+14
⇒0+14=14
Question 2
Sol :
(i)
sin245∘+cos245tan260
⇒(1√2)2+(1√2)2(√3)2
⇒12+123
⇒1+12⋅3
⇒26
⇒13
(ii)
sin30∘−sin90∘+2cos0∘tan30∘×tan60∘
⇒12−1+2(1)1√3×√3
⇒12+1
⇒1+22
⇒32
(iii)
⇒43tan230∘+sin260∘−3cos260∘+34tan260∘−2tan245∘
⇒43(1√3)2+(√32)2−3(12)2+34(√3)2−2⋅(1)2
⇒43⋅13+34−34+94−2
⇒49+94−2
⇒4×4+9×9−2×3636=16+81−1236
⇒2536
Question 3
(i) sin60∘cos245∘−3tan30∘+5cos90∘
⇒√3/2(1/√2)2−31√3+5(0)
⇒√3212−3√3+0
⇒√3−3√3
⇒√3⋅√3−3√3
⇒3−3√3
⇒0
(ii)
2√2cos45∘cos60∘+2√3sin30∘tan60∘−cos0∘
⇒2√21√2⋅12+2√3⋅12⋅√3−1
⇒1+3-1
⇒3
(iii)
45tan260∘−2sin230∘−34tan230∘
⇒45(√3)2−2(12)2−34(1√3)2
⇒45⋅3−2(14)−34⋅13
⇒125−8−14
⇒12×4−8×5×4−1×55×4
⇒48−160−520
⇒−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
RHS⇒2tanx1−tan2x
⇒2⋅tan30∘1−tan230
⇒2⋅1√31−(1√3)2
⇒2√31−13
⇒2√33−13
⇒2√323
⇒3√3√3√3
⇒3√33
⇒√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°
⇒4⋅12⋅12⋅1
⇒1
∴LHS=RHS
Question 6
Sol :
(i)
√1−cos230∘1−sin230∘ (∵sin2A+cos2A=1)
⇒√sin230cos230∘
⇒√(12)2(√32)2
⇒√1434
⇒1√3
(ii)
⇒sin45∘⋅cos45∘⋅cos60∘sin60∘cos30∘⋅tan45∘
⇒1√2⋅1√2⋅12√32⋅√32⋅1
⇒1232
⇒13
Question 7
Sol :
Given : θ=30°
(i)
LHS⇒sin2θ=sin2.30∘
=sin 60
=√32
RHS⇒2sinθcosθ=2⋅sin30∘⋅cos30∘
=2⋅12⋅√32
=√32
∴LHS=RHS
(ii)
LHS
⇒cos2θ=cos2.30∘
=cos60∘
=12
RHS
⇒2cos2θ−1=2⋅cos2⋅30∘−1
=2⋅(√32)2−1
=2√34−1
=3−22
=12
∴LHS=RHS
(iii)
LHS⇒sin3θ
=sin3×30∘
=sin90∘
=1
RHS
⇒3sinθ−4sin3θ
⇒3⋅sin30∘−4sin330∘
⇒3⋅12−4(12)3
⇒32−48
⇒3−12
⇒22=1
∴LHS=RHS
(iv)
LHS⇒cos3θ⇒cos3×30°
⇒cos 90°
⇒0
RHS⇒4cos3θ−3cosθ
⇒4⋅cos330∘−3cos30∘
⇒4⋅(√32)3−3⋅√32
⇒4⋅3√38−3√32
⇒3√32−3√32
⇒0
∴LHS=RHS
Question 8
Sol :
Given : θ=30°
⇒2sinθ : sin2θ
⇒2sinθsin2θ=2⋅sin30∘sin2×30∘
=2×12sin60
=1√32
2sinθsin2θ=2√3
∴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°
⇒√32⋅√32+12⋅12
⇒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°
⇒12⋅√32−√32⋅12
⇒√34−√34
⇒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°
⇒√32⋅√32−12.12
⇒32−12
⇒3−12=12=RHS
∴sin(A-B)=sinAcosB-cosAsinB
(iv)
A=60° ; B=30°
LHS⇒tan(A-B)=tan(60°-30°)
=tan 30°
=1√3
RHS⇒tanA−tanB1+tanA⋅tanB
⇒lan60∘−tan30∘1+tan60∘tan30∘
⇒√3−1√31+√31√3
⇒√3⋅√3−1√32
⇒3−1√32
⇒2√32
⇒1√3
∴LHS=RHS
i.e. tan(A−B)=tanA−tanB1+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=30−20
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+sin2θ−1
⇒2⋅tan245+sin245−1
⇒2⋅(1)+(1√2)2−1
⇒2+12−1
⇒4+1−22
⇒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+sin2θ−1
⇒2tan245+sin245−1
⇒2⋅(1)+(1√2)2−1
⇒2+12−1
⇒4+1−22
⇒32
Question 13
Sol :
(i) From figure
tanx∘=√31
tanx∘=√3
(ii) x=60°
Question 15
Sol :
Given : tan3x=sin45∘⋅cos45∘+sin30∘
tan3x=1√2⋅1√2+12
=12+12
=1+12
=22
tan 3x=1
tan 3x=tan 45°
3x=45°
x=15°
Question 16
Sol :
(i) Given : 4cos2x−1=0
4cos2x=1
cos2x=14
cos3x=1√4
cosx=12
cos x=cos 60°
x=60°
(ii) sin2x+cos2x⇒sin260∘+cos260∘
⇒(√32)2+(12)2
⇒34+14
⇒3+14
⇒44
=1
(iii) cos2x∘−sin2x∘
⇒cos260∘−sin260∘
⇒(12)2−(√32)2
⇒14−34
⇒1−34
⇒−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=90∘3
x=30∘
(i) sinx⇒sin30=12
(ii) cos2x⇒cos 2×30°
⇒cos 60°
⇒12
(iii) tan2x−sec2x=tan230∘−sec230∘
=(1√3)2−(2√3)2
=13−43
=1−43=−33
=-1
Question 19
Sol :
Given : 3tan2θ−1=0
3tan2θ=1
tan2θ=13
tanθ=1√3
θ=30∘
∴cos2θ=cos2×30°
=cos60°
=12
Question 20
Sol :
Given : sinx+cosy=1
∵x=30°
∴sin30°+cosy=1
12+cosy=1
cosy=1−12
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=60∘A−B=30∘2A=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=8⋅√32
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=6⋅√32
AC=3√3
From Figure
Question 24
Question 25
ΔABC ; ∠B=45°
AD=4(3−√3)
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