|a+b|<=|a|+|b|
|f(x)-f(m)|=|x^2-m^2-2(x-m)|=|(x-m)(x+m+2)|=|(x-m)|*|(x+m+2)|
<3|x+m+2|=3|x-m+2m+2|
3|x-m+2m+2|<=3|x-m|+3|2m+2|<=9+3|2m+2|
9+3|2m+2|<=9+3|2m|+3*2=6|m|+15
所以|f(x)-f(m)|〈6|m|+15