三角函数的转换问题!y=sina【2(x+π/4)+π/3】=cos(2x+π/3)这是怎么转换的)急!

解y=sin[2(x+π/4)+π/3]=sin(2x+π/2+π/3)=sin[π/2+(2x+π/3)]=sinπ/2cos(2x+π/3)+cosπ/2sin(2x+π/2)=cos(2x+π/3)其实是有一条公式的sin[π/2+a)=cosa∴上面那步sin[π/2+(2x+π/3)]直接等于cos(2x+π/3)