sina+sinb+2sinasinbcos(a+b)=sina+sinb+2sinasinb(cosacosb-sinasinb) =sina+sinb+2sinasinbcosacosb-2sinasinb =sina-sinasinb+sinb-sinasinb+2sinasinbcosacosb =sina(1-sinb)+sinb(1-sina)+2sinasinbcosacosb =sinacosb+sinbcosa+2sinasinbcosacosb =(sinacosb+sinbcosa) =sin(a+b) 证毕,