ok heres a question i cant figure out why the answer is the way it is.(adsbygoogle = window.adsbygoogle || []).push({});

Solve each equation for all values of x.

1.cos2x=cosx

*First i use the double identity 2cos²x-1 for cos2x and got:

2cos²x-1=cosx

*then i subtracted cosx and got:

2cos²x-cosx-1=0

*then i subtract 1 on both side and got:

2cos²x-cosx=1

*I then factor 2cos²x-cosx and got:

cosx(2cosx-1)=1

*then i set them equal to 1 and got this:

cosx=1

=0°

2cosx-1=1

added one and divided by 2 to get:

cosx=1

=0°

Since it is cosine i set it like this: 0°+360k°

But my book says the correct answer is 0°+120k°

Please someone help me with this.

And i may have more questions on some other double identity problems.

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# Homework Help: Trig double-angle itdentities

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