Trừ các vế cho 1
=> 2a+b+c+d/a -1 =a+2b+c+d/b -1 =a+b+2c+d/c -1 =a+b+c+2d/d -1
=>a+b+c+d/a=a+2+c+d/b=a+b+c+d/c=a+b+c+d/d
+) Xét a+b+c+d = 0
=> a+b = -(c+d)
b+c = - (a+d)
c+d = - (a+b)
a+d = - (b+c)
=> M = -1 +-1 + -1 +-1 = 4
+) xÉT A+B+C+D khác 0
=> a = b = c = d
=> M = 1+1+1+1 = 4