1-cosx=1-(1-2sin(1/2x)^2)=2sin(1/2x)^2limx->01-cosx=limx->02sin(1/2x)^2 =limx->02(1/2x)^2 =2×1/4x^2 =1/2x^2limx→0(1-cosx)/x^2=1/...1-cosx是等于2sin(x/2)^2还是2sin(1/2x)^2呢？？错了，应该是1-cosx=1-[1-2sin²(x/2)]=2sin²(x/2)