SOLUTION: I need to simplify 2cos^2(θ/3)-1 using a double-angle formula. I really can't figure it out, thank you!

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Question 1086276: I need to simplify 2cos^2(θ/3)-1 using a double-angle formula. I really can't figure it out, thank you!
Answer by Edwin McCravy(20062) About Me  (Show Source):
You can put this solution on YOUR website!
2cos%5E2%28theta%2F3%29-1

The double angle formula to use is this one

        cos%282alpha%29+=+2cos%5E2%28alpha%29-1

We substitute theta%2F3 for alpha

        cos%282%2Aexpr%28theta%2F3%29%29+=+2cos%5E2%28theta%2F3%29-1

The right side of that equation is what we're given,
so the answer is the left side:

        cos%282%2Aexpr%28theta%2F3%29%29

which can be also be written as

       cos%28%282theta%29%2F3%29%29 or

       cos%28expr%282%2F3%29%2Atheta%29

Edwin