cos a-cos b =-2*cos(a+b)/2 *sin(a-b)/2|cos a-cos b |=2*|cos(a+b)/2|*|sin(a-b)/2|≤2|sin(a-b)/2|又:sinx≤x所以sin(a-b)/2≤(a-b)/2所以2|sin(a-b)/2|≤2*|(a-b)/2|=|a-b|至于sinx≤x 是可以证明的....