f(x)=-√x,设x1>x2>=0,则
f(x1)-f(x2)=-√x1+√x2=√x2-√x1
=(x2-x1)/(√x2+√x1)
∵x1>x2>=0
∴x2-x10
从而(x2-x1)/(√x2+√x1)