f(t)-f(s)t-s≤f(u)-f(s)u-s≤f(u)-f(t)u-t for convex fIf f is convex in (a,b) and if a<s<t<u<b, thenf(t)-f(s)t-s≤f(u)-f(s)u-s≤f(u)-f(t)u-t.(1)