f(x)=x²+1/x²设x1>x2≥0[f(x1)-f(x2)]/(x1-x2)=(x1²+1/x1²-x2²-1/x2²)/(x1-x2)=[(x1-x2)(x1+x2)+(1/x1-1/x2)(1/x1+1/x2)]/(x1-x2)={(x1-x2)(x1+x2)+[(x2-x1)/(x1x2)][(x2+x1)/(x1x2)]...为什么要除x1-x2？便于运算