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Hi,

The identity

[itex]sin(u) + sin(v) = 2 * sin (\frac{u+v}{2}) * cos(\frac{u-v}{2})[/itex]

http://en.wikipedia.org/wiki/List_of_trigonometric_identities#Product-to-sum_and_sum-to-product_identities

Does not always work. I put the equation :

[itex](sin(u) + sin(v)) - (2 * sin (\frac{u+v}{2}) * cos(\frac{u-v}{2}))[/itex]

With u equal to -4.1 and v equal to 99 into wolfram alpha and it gave me the answer -1.11022x10^-16

http://www.wolframalpha.com/input/?i=x%3D%28sin%28-4.1%29%2Bsin%2899%29%29-%28%282*sin%28%28-4.1%2B99%29%2F2%29*cos%28%28-4.1-99%29%2F2%29%29%29

If the identity is true, shouldn't the answer always be 0?

What's going on here?

Thanks

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# Sum to Product Trigonometric identity does not work

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