SOLUTION: Find the solutions of the equation that are in the interval [0, 2pi). cos u + cos 2u = 0

Question 843806: Find the solutions of the equation that are in the interval [0, 2pi).
cos u + cos 2u = 0
Answer by lwsshak3(11628) (Show Source): You can put this solution on YOUR website!
Find the solutions of the equation that are in the interval [0, 2pi).
cos u + cos 2u = 0
cosu+cos^2u-sin^2u=0
cosu+cos^2u-(1-cos^2u)=0
cosu+cos^2u-1+cos^2u=0
2cos^2u+cosu-1=0
(2cosu-1)(cosu+1)=0
cosu=1/2
u=π/3,5π/3
or
cosu=-1
u=π
solutions: π/3, 5π/3, π
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