Write the following sums using sigma notation:

1. $$7+9+11+\dots +101$$
2. $$\frac{1}{2}+\frac{2}{3}+\frac{3}{4}+\dots +\frac{100}{101}$$
3. $$-1+\frac{1}{4}-\frac{1}{9}+\frac{1}{16}-\dots +\frac{1}{256}$$

For part (a), start by writing

Note that odd numbers can be expressed in the form $2k+1$

For part (c), you can use $(-1{)}^k$ to make the sign of each term alternate