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91Ó°ÊÓ

In Problems 69-78, solve each equation on the interval 0≤θ<2π.

cos2θ=cosθ.

Short Answer

Expert verified

The solution set for the equationcos2θ=cosθis,0,2π3,4π3.

Step by step solution

01

Step 1 Given equation is,

cos2θ=cosθ.

The double angle formula for cos2θis, cos2θ=2cos2θ-1

Substitute the known values in the given equation.

2cos2θ-1=cosθ2cos2θ-1-cosθ=0

02

Step 2 Factor the expression.

cosθ-12cosθ+1=0cosθ-1=0or2cosθ+1=0cosθ=1or2cosθ=-1cosθ=1orcosθ=-12

In the interval [0,2π)cosθhas value 1for θ=0and has value -12for θ=2π3,4π3.

Therefore the solution set is,0,2Ï€3,4Ï€3.

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