题目

Schools across China are expected to hire 50,000 college graduates this year as short-term teachers        increase job opportunities.        A．help  B．to have helped  C．to help      D．having helped 答案：C18．已知函数f（x）=cos2（x-$\frac{π}{6}$）-cos2x，x∈R（Ⅰ）求f（x）的最小正周期；（Ⅱ）求y=f（x）在区间$[{-\frac{π}{3}，\frac{π}{4}}]$上的最大值和最小值．