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Question Number 6370 by sanusihammed last updated on 25/Jun/16

If  x = ((3sinθ)/(√(x^2  + y^2  )))   and  y = 4cosθ  find  cosθ

$${If}\:\:{x}\:=\:\frac{\mathrm{3}{sin}\theta}{\sqrt{{x}^{\mathrm{2}} \:+\:{y}^{\mathrm{2}} \:}}\:\:\:{and}\:\:{y}\:=\:\mathrm{4}{cos}\theta \\ $$$${find}\:\:{cos}\theta \\ $$

Commented by nburiburu last updated on 25/Jun/16

cosθ=y/4 with 16x^2 (x^2 +y^2 )+9y^2 =144

$${cos}\theta={y}/\mathrm{4}\:{with}\:\mathrm{16}{x}^{\mathrm{2}} \left({x}^{\mathrm{2}} +{y}^{\mathrm{2}} \right)+\mathrm{9}{y}^{\mathrm{2}} =\mathrm{144} \\ $$

Commented by sanusihammed last updated on 25/Jun/16

What is the final answer.

$${What}\:{is}\:{the}\:{final}\:{answer}. \\ $$

Commented by prakash jain last updated on 25/Jun/16

sin θ=a  cos θ=b  You have followinh equations  x=((3a)/(√(x^2 +y^2 )))  y=4b  a^2 +b^2 =1  So with 4 variables you have only 3 equations.  The information is not complete.

$$\mathrm{sin}\:\theta={a} \\ $$$$\mathrm{cos}\:\theta={b} \\ $$$$\mathrm{You}\:\mathrm{have}\:\mathrm{followinh}\:\mathrm{equations} \\ $$$${x}=\frac{\mathrm{3}{a}}{\sqrt{{x}^{\mathrm{2}} +{y}^{\mathrm{2}} }} \\ $$$${y}=\mathrm{4}{b} \\ $$$${a}^{\mathrm{2}} +{b}^{\mathrm{2}} =\mathrm{1} \\ $$$$\mathrm{So}\:\mathrm{with}\:\mathrm{4}\:\mathrm{variables}\:\mathrm{you}\:\mathrm{have}\:\mathrm{only}\:\mathrm{3}\:\mathrm{equations}. \\ $$$$\mathrm{The}\:\mathrm{information}\:\mathrm{is}\:\mathrm{not}\:\mathrm{complete}. \\ $$

Commented by FilupSmith last updated on 25/Jun/16

∴ x = ((3sinθ)/(√(x^2 +4^2 cos^2 θ)))  ∴ x^2 (x+16cos^2 θ) = 3sinθ  x^3 +16x^2 (1−sin^2 θ)=3sinθ  x^3 +16x^2 −16x^2 sin^2 θ=3sinθ  continue to transpose for θ  and solve cosθ

$$\therefore\:{x}\:=\:\frac{\mathrm{3sin}\theta}{\sqrt{{x}^{\mathrm{2}} +\mathrm{4}^{\mathrm{2}} \mathrm{cos}^{\mathrm{2}} \theta}} \\ $$$$\therefore\:{x}^{\mathrm{2}} \left({x}+\mathrm{16cos}^{\mathrm{2}} \theta\right)\:=\:\mathrm{3sin}\theta \\ $$$${x}^{\mathrm{3}} +\mathrm{16}{x}^{\mathrm{2}} \left(\mathrm{1}−\mathrm{sin}^{\mathrm{2}} \theta\right)=\mathrm{3sin}\theta \\ $$$${x}^{\mathrm{3}} +\mathrm{16}{x}^{\mathrm{2}} −\mathrm{16}{x}^{\mathrm{2}} \mathrm{sin}^{\mathrm{2}} \theta=\mathrm{3sin}\theta \\ $$$$\mathrm{continue}\:\mathrm{to}\:\mathrm{transpose}\:\mathrm{for}\:\theta \\ $$$${and}\:{solve}\:\mathrm{cos}\theta \\ $$

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