Rewrite the following sums using summation notation.

1. $5 + 7 + 9 + 11 + 13$

   Hint (a):

   Answers vary. Here are three possible answers.
   Notice the index of all answers: every index has exactly $5$ terms... because there are exactly $5$ terms in the series.

2. $2\text{ cos }2π + 3\text{ cos }3π + 4\text{ cos }4π + 5\text{ cos }5π$

   Hint (b):

   Complete the argument:
   $\displaystyle\sum_{n=2}^{5}$

3. $\frac { 1 } { 5 } f ( - 2 ) + \frac { 1 } { 5 } f ( - 1 ) + \frac { 1 } { 5 } f ( 0 ) + \frac { 1 } { 5 } f ( 1 ) + \frac { 1 } { 5 } f ( 2 )$

   Hint (c):

   Complete the index: