I haven't looked at the whole program yet, but the slope calculations are 
missing parentheses.

>slopeAB = yB-yA/xB-xA                #calculates slope of side AB
>slopeBC = yC-yB/xC-xB                #calculates slope of side BC
>slopeCD = yD-yC/xD-xC                #calculates slope of side CD
>slopeDA = yA-yD/xA-xD                #calculates slope of side DA
>

should be: slopeAB = ( yB - yA ) / ( xB - xA )                #calculates 
slope of side AB