1. Solve the following equations for 0∘≤θ≤360∘. (a) 3cosθ−sinθ=2
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(a) 3cosθ−sinθ=2 Comparing with a cosθ−bsinθ=c, a=3,b=1,c=2∴ √a2+b2=√32+12=√10 Let tanα=ba⇒tanα=13∴ α=18∘26′∴ 3cosθ−sinθ=2⇒√10cos(θ+18∘26′)=2∴ cos(θ+18∘26′)=0.6325∴ θ+18∘26′=50∘46′ (or) θ+18∘26′=360∘−50∘46′∴ θ=32∘20′ (or) θ=290∘48′
(b) 2sinθ−3cosθ=3
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(b) 2sinθ−3cosθ=3 Comparing with a sinθ−bcosθ=c, a=2,b=3,c=3∴ √a2+b2=√22+32=√13 Let tanα=ba⇒tanα=32∴ α=56∘19′∴ 2sinθ−3cosθ=3⇒√13sin(θ−56∘19′)=3∴ sin(θ−56∘19′)=0.8321∴ θ−56∘19′=56∘19′ (or) θ−56∘19′=180∘−56∘19′∴ θ=112∘38′ (or) θ=180∘
(c) cosθ+2sinθ=2
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(c) cosθ+2sinθ=2 Comparing with acosθ+bsinθ=c, a=1,b=2,c=2∴ √a2+b2=√12+22=√5 Let tanα=ba⇒tanα=2∴ α=63∘26′∴ cosθ+2sinθ=2⇒√5cos(θ−63∘26′)=2∴ cos(θ−63∘26′)=0.8944∴ θ−63∘26′=26∘34′ (or) θ−63∘26′=360∘−26∘34′ ∴ θ=90∘ (or) θ=396∘52′
(d) 8sinθ+6cosθ=5
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(d) 8sinθ+6cosθ=5 Comparing with asinθ+bcosθ=c, a=8,b=6,c=5∴ √a2+b2=√82+62=10 Let tanα=ba⇒tanα=34∴ α=36∘52′∴ 8sinθ+6cosθ=5⇒10sin(θ+36∘52′)=5∴ sin(θ+36∘52′)=0.5∴ θ+36∘52′=30∘ (or) θ+36∘52′=150∘ (or) θ+36∘52′=390∘∴ θ=−6∘52′ (or) θ=113∘8′ (or) θ=353∘8′ Since 0∘≤θ≤360∘,θ=−6∘52′ is impossible.∴ θ=113∘8′ (or) θ=353∘8′
(e) √3cosθ+sinθ=√2
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(e) √3cosθ+sinθ=√2 Comparing with acosθ+bsinθ=c, a=√3,b=1,c=√2∴ √a2+b2=√3+12=2 Let tanα=ba⇒tanα=1√3∴ α=30∘∴ √3cosθ+sinθ⇒2cos(θ−30∘)=√2∴ cos(θ−30∘)=√22∴ θ−30∘=45∘ (or) θ−30∘=315∘ ∴ θ=75∘ (or) θ=345∘
(f) sinθ−cosθ=√2
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(f) sinθ−cosθ=√2 Comparing with asinθ−bcosθ=c, a=1,b=1,c=√2∴ √a2+b2=√12+12=√2 Let tanα=ba⇒tanα=1∴ α=45∘∴ sinθ−cosθ=√2⇒√2sin(θ−45∘)=√2∴ sin(θ−45∘)=1∴ θ−45∘=90∘ ∴ θ=135∘
(g) 4sinθ+3cosθ=0
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(g) 4sinθ+3cosθ=0 Comparing with asinθ+bcosθ=c, a=4,b=3,c=2∴ √a2+b2=√42+32=5 Let tanα=ba⇒tanα=34∴ α=36∘52′∴ 4sinθ+3cosθ=0⇒5sin(θ+36∘52′)=0∴ sin(θ+36∘52′)=0∴ θ+36∘52′=0∘ (or) θ+36∘52′=180∘ (or) θ+36∘52′=360∘∴ θ=−36∘52′ (or) θ=143∘8′ (or) θ=323∘8′ Since 0∘≤θ≤360∘,θ=−36∘52′ is impossible.∴ θ=143∘8′ (or) θ=323∘8′
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