Q. Find all angles, 0∘≤θ<360∘, that satisfy the equation below, to the nearest tenth of a degree.9cos2θ+3cosθ=8cosθ+4Answer: θ=
Rewrite Equation: Rewrite the given equation to collect all terms on one side and set the equation to zero.9cos2(θ)+3cos(θ)−8cos(θ)−4=0Simplify the equation by combining like terms.9cos2(θ)−5cos(θ)−4=0
Simplify Equation: Factor the quadratic equation in terms of cos(θ).$9cos2(θ)−5cos(θ)−4 = 3cos(θ)+13cos(θ)−4\)Set each factor equal to zero to find the values of cos(θ).3cos(θ)+1=0 and 3cos(θ)−4=0
Factor Quadratic Equation: Solve each equation for cos(θ). For 3cos(θ)+1=0: cos(θ)=−31 For 3cos(θ)−4=0: cos(θ)=34 However, since the cosine of an angle cannot be greater than 1, cos(θ)=34 is not a valid solution.
Solve for cos(θ): Find the angles θ that correspond to cos(θ)=−31 in the range 0^\circ \leq \theta < 360^\circ. Use the inverse cosine function and consider the symmetry of the cosine function, which is positive in the first and fourth quadrants and negative in the second and third quadrants. θ=cos−1(−31) Calculate the principal value. θ≈cos−1(−31)≈109.5∘
Find θ for cos(θ)=−31: Find the second angle in the range 0^\circ \leq \theta < 360^\circ that also has a cosine of −31.Since cosine is negative in the second and third quadrants, the second angle is the supplement of the principal value.θ=180∘+(180∘−109.5∘)θ≈180∘+70.5∘θ≈250.5∘
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