the sum is still cosα+cosβ=2cos[(α+β)/2]·cos[(α-β)/2]

the sum of sai sai is sinα+sinβ=2sin[(α+β)/2]·cos[(α-β)/2]

the difference between mouth and mouth is loss cosα-cosβ=-2sin[(α+β)/2]·sin[(α-β)/2]

the difference is sinα-sinβ=2cos[(α+β)/2]·sin[(α-β)/2]