MN=MC+CD+DN,
MC=1/2*AC,
CD=2/7*AB,
DN=1/4*DB,
MN=1/2*AC+2/7*AB+1/4*DB,
因为AC=1/7*AB,DB=4/7*AB
则有MN=1/2*AC+2/7*AB+1/4*DB
=1/14*AB+2/7*AB+1/7*AB
=1+4+2
=7
MN=MC+CD+DN,
MC=1/2*AC,
CD=2/7*AB,
DN=1/4*DB,
MN=1/2*AC+2/7*AB+1/4*DB,
因为AC=1/7*AB,DB=4/7*AB
则有MN=1/2*AC+2/7*AB+1/4*DB
=1/14*AB+2/7*AB+1/7*AB
=1+4+2
=7