It was the last day of the final examination in a university. A group of students entered the classroom happily. This was their last exam. They knew it would be easy. The professor had said that they could bring any book or note they wanted. The only requirement(要求) was that they couldn't talk with each other.
   The professor handed out the papers. They were glad to find there were only five questions. Three hours passed. The professor asked, "How many of you have finished all the five questions?" None of the students answered."How many of 3'ou have finished four?" No answers again. "Thee? Two?" The students moved restlessly (不安地) in their seats."One, then? Certainly somebody has finished  one." But the class remained silent. "That is exactly what I expected," the professor said. "I just want to teach you a lesson--although you have completed four years of study, there are still many things you don't know about." Then be added, "You will all pass the exam, But remember: even though you are graduating from college, your education has just begun."

已知函数$f\left(x\right)=a{x}^3+bx,(ab\ne 0)$,对任意$x_1,x_2\in R,x_1\ne x_2$都有$\frac{f\left(x_1\right)-f\left(x_2\right)}{x_1-x_2}>0$，若$m+n<0$，则$f\left(m\right)+f\left(n\right)$的值（ ）

黄土高原特有的传统民居是（　　）