$50$ points$)$ find simplified version $($with the sum or product symbols$)$ of CSOP and CPOS

forms for the following functions:

a$)(20$ points$)$ F $($ a $,$b $,$c $)=$ab$+$ a$\text{'}$ c

b$)(30$ points$)$ F $($ A $,$ B $,$C $,$ D $)=($ A$+$ B $\text{'})($C$+$ D$)($ A $\text{'}+$ B$+$ D $\text{'})$