两角和的余弦公式:cos(A+B)=cosAcosB-sinAsinB
∴ 根号2cosx-根号6sinx
=2√2*[(1/2)*cosx-(√3/2)sinx]
=2√2[cosx*cos(π/3)-sinx*sin(π/3)]
=2√2cos(x+π/3)