用定义法：令1≤x1＜x2f(x2)-f(x1) = (x2² + 1/x2) - (x1² + 1/x1)= (x2²- x1²) - ( 1/x1 - 1/x2)= (x2+x1)(x2-x1) - (x2-x1)/(x1x2)= (x2-x1){x2+x1-1/(x1x2)}∵1≤x1＜x2∴(x2-x1)＞0∵1≤x1...(x2² + 1/x2) - (x1² + 1/x1)
=(x2²- x1²) - ( 1/x1 - 1/x2) 请问这是怎么回事，谢谢！加法结合律嘛！
(A+B)-(C+D)
= (A-C)-(D-B)