- #1

needingtoknow

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## Homework Statement

I have to prove that sin^2(a/2) = (1-cosa)/(2)

cosa = cos(2*(a/2)) = 1 -2sin^2(a/2)

I don't understand this step that was given in the solutions how do I get it?

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- Thread starter needingtoknow
- Start date

- #1

needingtoknow

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I have to prove that sin^2(a/2) = (1-cosa)/(2)

cosa = cos(2*(a/2)) = 1 -2sin^2(a/2)

I don't understand this step that was given in the solutions how do I get it?

- #2

tia89

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- #3

needingtoknow

- 160

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- #4

tia89

- 116

- 2

Now assuming b=a/2, you have cos(2b)=cos^2(b)-sin^2(b) indeed... now you can use the fundamental formula cos^2(b)+sin^2(b)=1 and express cos(2b) only as function of the sin^2(b). Done this, all is straightforward replacing b with a/2

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