0/0型，罗必塔法则
原式=lim[(1-2sinx)+(x-π/6)*(-2cosx)]/[2arctan(x-π/6)*1/[1+(x-π/6)²]]
=lim[(1-2sinx)+(x-π/6)*(-2cosx)]*[1+(x-π/6)²]/2arctan(x-π/6)
=lim[1-2sinx-2cosx*(x-π/6)]/2arctan(x-π/6)
=lim[-2cosx+2sinx*(x-π/6)-2cosx]/[2/1+(x-π/6)²]
=lim[-4cosx+2sinx*(x-π/6)]*[1+(x-π/6)²]/2
=lim-2cosx+sinx*(x-π/6)*[1+(x-π/6)²]
=-2cos(π/6)
=-√3
