sin(a+b)=sinacosb+cosasinb sin(a-b)=sinacosb-cosasinb
两å¼ç¸å å¾ï¼sinacosb=1/2[sin(a+b)+sin(a-b)](1)
两å¼ç¸åå¾ï¼cosasinb=1/2[sin(a+b)-sin(a-b)](2) cos(a+b)=cosacosb-sinasinb
cos(a-b)=cosacosb+sinasinb 两å¼ç¸å å¾ï¼ cosacosb=1/2[cos(a+b)+cos(a-b)](3)
两å¼ç¸åå¾ï¼sinasinb=-1/2[cos(a+b)-cos(a-b)](4)
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sina+sinb=2sin[(a+b)/2]*cos[(a-b)/2] å
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