题目

After years of hearing drivers complain about scratches on their cars, Japan’s Nissan Motor Company has officially announced the next big thing — a paint that not only resists scratches and scrapes, but actually repairs itself within a few days. The new material, developed by Nippon Paint Company, contains an elastic rubbery-like resin (弹性树脂) that is able to heal minor marks caused by car wash equipment, parking lot encounters, road debris (石头碎片) or even on-purpose destruction.     The automaker admits its results vary depending on the temperature and the depth of the da12．已知函数f（x）=$\frac{x}{{x}^{2}+1}$，证明f（x）在区间[1，+∞）上为减函数．