BC=AC-AB=AC-2DC=AC-2(AC-AD)= -AC+2AD= -a+2b ;
MN=AN-AM=1/2*AB-1/2*(AC+AD)=DC-1/2*(AC+AD)
=AC-AD-1/2*(AC+AD)=1/2*AC-3/2*AD=1/2*a-3/2*b .
BC=AC-AB=AC-2DC=AC-2(AC-AD)= -AC+2AD= -a+2b ;
MN=AN-AM=1/2*AB-1/2*(AC+AD)=DC-1/2*(AC+AD)
=AC-AD-1/2*(AC+AD)=1/2*AC-3/2*AD=1/2*a-3/2*b .