Tính M = a + b/ c + d + b + c/ d + a + c + d/ a + b + a + d/ b + c

cho 2a+b+c+d/a = a+2b+c+d/b = a+b+2c+d/c = a+b+c+2d/d

Tính M= a+b/c+d + b+c/d+a + c+d/a+b + a+d/b+c