Trigonometric function and differential product formula
\(cosα+cosβ=2cos[(α+β)/2]·cos[(α-β)/2]\)
\(sinα+sinβ=2sin[(α+β)/2]·cos[(α-β)/2]\)
\(cosα-cosβ=-2sin[(α+β)/2]·sin[(α-β)/2]\)
\(sinα-sinβ=2cos[(α+β)/2]·sin[(α-β)/2]\)
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