设x1>x2,
f(x1)-f(x2)=sqrt(x1^2+1)-sqrt(x2^2+1)-a(x1-x2)
=(x1-x2)/[sqrt(x1^2+1)+sqrt(x2^2+1)]-a(x1-x2)
=(x1-x2){1-a[sqrt(x1^2+1)+sqrt(x2^2+1)]}/[sqrt(x1^2+1)+sqrt(x2^2+1)]
设x1>x2,
f(x1)-f(x2)=sqrt(x1^2+1)-sqrt(x2^2+1)-a(x1-x2)
=(x1-x2)/[sqrt(x1^2+1)+sqrt(x2^2+1)]-a(x1-x2)
=(x1-x2){1-a[sqrt(x1^2+1)+sqrt(x2^2+1)]}/[sqrt(x1^2+1)+sqrt(x2^2+1)]