lim(x→0)【(√4-x^2-2)/(1-cos2x)】=lim(x→0)(4-x^2-4)/(4-x^2+4)(1-cos2x) 分母有理化
=lim(x→0)-x^2/(4-x^2+4)2sinx^2 cos2x=1-2sinx^2
=lim(x→0)-x^2/16sinx^2
=-1/16 (x→0)时 x/sinx=1