求8,最好有过程, 

8、sin(π/12)=cos(5π/12)f(cosx)=cos2x,f[cos5π/12)]=cos(5π/6)=-cos(π/6)=-√3/2。9、π=π/2时,f(x)=cos2x+c是偶函数,最小正值是π/2。