1- cosX = 1 - [ 1 - 2(sinx/2)^2] = 2[sin(x/2)]^2
1-cosX/X^2 = 2[sin(x/2)]^2/x^2
= (1/2)* [sin(x/2)/(x/2)]^2
sin(x/2)/(x/2) 当x趋近于0时,其极限为1
（这是书上的基本定理）
因此
lim[(1-cosX)/x^2] = 1/2