求下列各式的值cos7π/12.(2）cos（-61π/12）利用公式Ca+β,Ca-β证明cos（a+π/2)=-sina(2)cos(-a+π/2)=sina

Answer 1

（1）cos（7π/12）=cos(π/2+π/12)=-sin(π/12)=-（√6-√2）/4
（2）cos（-61π/12）=cos(61π/12）=cos(13π/12）=cos(π+π/12）=-cos(π/12)=-（√6+√2）/4
cos(a+π/2)=cosacos(π/2)-sinasin(π/2)=cosa*0-sina*1=-sina
cos(-a+π/2)=cos(-a)cos(π/2)-sin(-a)sin(π/2)=cosa*0+sina*1=sina