x趋向0
lim(1-(cosx)^(1/2))/(x(1-cos(x)^(1/2))
=lim 1/x
=∞
lim(1-(cosx)^(1/2))/(x(1【+】cos(x)^(1/2))
=lim(1-cosx)/(x(1【+】cos(x)^(1/2)) ^2
=lim[2sin^(x/2)]/(4x)
=lim x/8
=0