1、正加正，正在前：sin α+sinβ=2sin[(α+β)/2]·cos[(α-β)/2]
2、正減正，餘在前：sin α-sin β=2cos[(α+β)/2]·sin[(α-β)/2]
3、餘加餘，餘并肩：cos α+cos β=2cos[(α+β)/2]·cos[(α-β)/2]
4、餘減餘，餘不見，負号很讨厭：cos α-cos β=-2sin[(α+β)/2]·sin[(α-β)/2]
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