# "Sum to Product" Trigonometric identity does not work

by CraigH
Tags: identity, sum to product, trigonometric
 P: 169 Hi, The identity $sin(u) + sin(v) = 2 * sin (\frac{u+v}{2}) * cos(\frac{u-v}{2})$ http://en.wikipedia.org/wiki/List_of...uct_identities Does not always work. I put the equation : $(sin(u) + sin(v)) - (2 * sin (\frac{u+v}{2}) * cos(\frac{u-v}{2}))$ 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...9%2F2%29%29%29 If the identity is true, shouldn't the answer always be 0? What's going on here? Thanks
 P: 172 It seems like a rounding error to me. You can verify that the identity is in fact always true by using the half angle and angle addition formulae.
 PF Patron Sci Advisor Thanks Emeritus P: 15,673 The problem is not that the formula doesn't work, but with the fact that your calculator is incapable of precisely calculating the sine or cosine of an angle.
P: 169

## "Sum to Product" Trigonometric identity does not work

Ah okay, thank you for answering.
One thing though... If wolfram alpha knows that it can only calculate the sine or cosine of an angle to a certain precision, shouldn't it give the final answer to that precision, or less, so that it avoids giving misleading answers like the one it gave me.
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