题目

Tayka Hotel De sal Where: Tahua, Bolivia How much: About $ 95 a night Why it’s cool: You’ve stayed at hotels made of brick or wood, but salt? That’s something few can claim. Tayka Hotel de Sal is made totally of salt—including the beds (though you’ll sleep on regular mattresses(床垫) and blankets). The hotel sits on the Salar de Uyuni, a prehistoric dried—up lake that’s the world’s biggest salt flat. Builders use the salt from the 4,633一square—mile flat to make the bricks, and glue them together with a paste of wet salt that hardens when it dries. When rain starts （1）写出函数f（x）=x2-8x+9在定义域内的单调递增和递减区间；（2）研究函数f（x）=x4-8x2+9在定义域内的单调性，写出它在定义域内的单调递增区间，并简要说明理由；（3）对函数f（x）=x2+bx+c和f（x）=x4+bx2+c（其中常数b＜0）作推广，使它们都是你所推广的函数的特例，并研究推广后函数的单调性，（只须写出结论，不必证明）