sin(90+a)
=sin90cosa+sinacos90
=cosa+0
=cosasin(90+a)=sin90cosa+sinacos90这一步怎么推导出来的？这是一公式两角和公式sin(a+b)=sinacosb+sinbcosasin(a-b)=sinacosb-sinbcosacos(a-b)=cosacosb+sinasinbcos(a+b)=cosacosb-sinasinb