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Mathematics

If 5 cos θ - 12 sin θ = 0, find the value of sin θ + cos θ2 cos θ - sin θ\dfrac{\text{sin θ + cos θ}}{\text{2 cos θ - sin θ}}.

Trigonometrical Ratios

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Answer

(i) Given,

⇒ 5 cos θ - 12 sin θ = 0

⇒ 5 cos θ = 12 sin θ

sin θcos θ=512\dfrac{\text{sin θ}}{\text{cos θ}} = \dfrac{5}{12}

⇒ tan θ = 512\dfrac{5}{12}.

(sin θ + cos θ)(2 cos θ - sin θ)[Dividing numerator and denominator by cos θ](sin θ + cos θ)cos θ(2 cos θ - sin θ)cos θsin θcos θ+cos θcos θ2 cos θcos θsin θcos θtan θ+12tan θ [tan θ=sin θcos θ]Substituting values we get512+125125+121224512171219121719.\Rightarrow \dfrac{\text{(sin θ + cos θ)}}{\text{(2 cos θ - sin θ)}} \\[1em] [\text{Dividing numerator and denominator by cos θ}] \\[1em] \Rightarrow \dfrac{\dfrac{\text{(sin θ + cos θ)}}{\text{cos θ}}}{\dfrac{\text{(2 cos θ - sin θ)}}{\text{cos θ}}} \\[1em] \Rightarrow \dfrac{\dfrac{\text{sin θ}}{\text{cos θ}} + \dfrac{\text{cos θ}}{\text{cos θ}}}{\dfrac{\text{2 cos θ}}{\text{cos θ}} - \dfrac{\text{sin θ}}{\text{cos θ}}} \\[1em] \Rightarrow \dfrac{\text{tan θ} + 1}{2 - \text{tan θ}} \space [\because \text{tan θ} = \dfrac{\text{sin θ}}{\text{cos θ}}] \\[1em] \text{Substituting values we get} \\[1em] \Rightarrow \dfrac{\dfrac{5}{12} + 1}{2 - \dfrac{5}{12}} \\[1em] \Rightarrow \dfrac{\dfrac{5 + 12}{12}}{\dfrac{24 - 5}{12}} \Rightarrow \dfrac{\dfrac{17}{12}}{\dfrac{19}{12}} \\[1em] \Rightarrow \dfrac{17}{19}.

Hence, (sin θ + cos θ)(2 cos θ - sin θ)=1719\dfrac{\text{(sin θ + cos θ)}}{\text{(2 cos θ - sin θ)}} = \dfrac{17}{19}.

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