cho a\(\ne\)-b,b\(\ne\)-c,c\(\ne\)-c. CMR:
\(\frac{b^2-c^2}{\left(a+b\right)\left(a+c\right)}\)+\(\frac{c^2-a^2}{\left(b+c\right)\left(b+a\right)}\)+\(\frac{-a^2-b^2}{\left(c+a\right)\left(c+b\right)}\)=\(\frac{b-c}{b+c}+\frac{c-a}{c+a}+\frac{a-b}{a+b}\)