cos 10 + cos 110 + cos 130

do you know what is the value of cos 10 + cos 110 + cos 130? 
according to mathematical calculations if it is equal to zero, how can you?explanation like this:The first time you have to remember that the sum of the commutative law applies, so nothing wrong if we do first is to calculate the value of cos 110 + cos 130 first.
(Remember that: cos (A + B) + cos (A - B) = 2.cos A.cos B) 
The value of cos 110 + cos 130 = 2.cos 120.cos10
                                   = 2. (-0.5). Cos 10
                                   = - Cos 10    
 from about cos 10 + cos 110 + cos 130 = cos 10 - cos 10
                                                               = 0
became evident that the cos 10 + cos 110 + cos 130 = 0 
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