Bài 1: Cho $A= \frac{x-y}{1+xy}$ ; $B= \frac{y-z}{1+yz}$ ; $C= \frac{z-x}{1+xz}$
CMR: $A+B+C=ABC$
Bài 2:
a) Cho a,b,c #0 và $\frac{1}{a}$ + $\frac{1}{b}$ + $\frac{1}{c}$=0
Tính $M$ = $\frac{bc}{a^2}$ + $\frac{ac}{b^2}$+ $\frac{ab}{c^2}$
b) Cho $\frac{a}{b+c}$+ $\frac{b}{c+a}$+$\frac{c}{a+b}$ =1
CMR: $\frac{a^2}{b+c}$+$\frac{b^2}{c+a}$+$\frac{c^2}{c+a}$$=0$