Express the following sums using sigma notation.

a$.89101112$

b$.24681012$

c$.1^6{2}^6{3}^6{4}^6$

d$.\frac{1}{4}\frac{1}{5}\frac{1}{6}\frac{1}{7}$

a$.89101112={\displaystyle \underset{-k=1}{\overset{5}{}}}(K7)$

b$.24681012={\displaystyle \underset{k=1}{\overset{6}{}}}$