Câu 7:($\frac{2a+b+c+d}{a}$ -1)=($\frac{a+2b+c+d}{b}$-1)=($\frac{a+b+2c+d}{c}$-1)=( $\frac{a+b+c+2d}{d}$-1)
*Nếu a+b+c+d=0
=>a+b=-(c+d)
b+c=-(a+d)
c+d=-(a+b)
d+a=-(b+c)
Thay vào ta có: M=$\frac{-(c+d)}{c+d}$+$\frac{-(d+a)}{d+a}$+$\frac{-(a+b)}{a+b}$+$\frac{-(b+c)}{b+c}$
=-1+-1+-1+-1=-4
*Nếu a+b+c+d #0 (#=khác)
=>a=b=c=d
=>1+1+1+1=4