SOLUTION: Prove the equation is not an identity by finding a value for x that defines both sides but are not equal: cos(3x)+cos(8x)=2cos(5x)cos(x)

SOLUTION: Prove the equation is not an identity by finding a value for x that defines both sides but are not equal: cos(3x)+cos(8x)=2cos(5x)cos(x)

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Question 1119802: Prove the equation is not an identity by finding a value for x that defines both sides but are not equal:
cos(3x)+cos(8x)=2cos(5x)cos(x)
Answer by Boreal(15235) (Show Source): You can put this solution on YOUR website!
Pick 90 degrees
cos (270)+cos (720) should not be equal to 2 cos (450)*cos (90).
Cos (270) is 0; cos 720 is 1, so left side is 1
cos 450 is 0 and cos 90 is 0, so right side is 0.
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