====== HW 2 ======
  * Read Ross section 7-9, especially p51-p54 where $\lim = \pm \infty$ are discussed. 
  * Do the exercises, but no submssions are required
    * 7.1, 7.2, 7.3
  * Submit the following problems
    * 7.4, 7.5(a)
    * 8.2（e), 8.4
    * 9.1 ( c) , 9.2(b), 9.3, 9.4, 9.9( c)