[[[1]]
易知,恒有:|x|≥|sinx|.x∈R.
两边平方,可得:
x²≥sin²x
∴可得:
(x/2)²≥sin²(x/2)
(x²/2)≥2sin²(x/2)
x²/2≥1-cosx
cosx≥1-(x²/2)