证明：
lim(x→0) (1-cosx) /(x^2 /2)
=lim(x→0) 2(1-cosx) /(x^2)
=lim(x→0) 2*2(sinx/2)^2 /(x^2)
=lim(x→0) [sin(x/2) ]^2 / (x/2)^2 令t=x/2
=lim(t→0) (sin t / t)^2
=1
所以：
x→0,1-cos x^2/2