a, Cho \(a\ne b\). Chứng minh \(\frac{1}{x-a}+\frac{1}{x-b}=\frac{1}{a}+\frac{1}{b}\) với \(x=\frac{2ab}{a+b}\)
b. Cho \(x=\frac{a-b}{a+b};\)\(y=\frac{b-c}{b+c};\)\(z=\frac{c-a}{c+a}.\)Chứng minh : \(\left(1-x\right)\left(1-y\right)\left(1-z\right)=\left(1+x\right)\left(1+y\right)\left(1+z\right)\)