Ta có .......................................
= abc/ac + bc = abc/ ab + ac = abc/bc + ba
suy ra ac + bc= ab+ ac = bc+ba
suy ra bc = ab = ac
suy ra c = b = a
suy ra M = 1
= abc/ac + bc = abc/ ab + ac = abc/bc + ba
=> ac + bc= ab+ ac = bc+ba
=>bc = ab = ac
=> c = b = a
=> M = 1