Có:
[tex]\frac{a^{2}}{b+c}+\frac{b^{2}}{a+c}+\frac{c^{2}}{a+b}=a(\frac{a}{b+c})+(\frac{b}{a+c})+c(\frac{c}{a+b})=a(\frac{a}{b+c}+1-1)+(\frac{b}{a+c}+1-1)+c(\frac{c}{a+b}+1-1)=a(\frac{a+b+c}{b+c}-1)+b(\frac{a+b+c}{a+c}-1)+c(\frac{a+b+c}{b+a}-1)=a.\frac{a+b+c}{b+c}+b.\frac{a+b+c}{a+c}+c.\frac{a+b+c}{b+a}-a-b-c=(a+b+c)(\frac{a}{b+c}+\frac{b}{a+c}+\frac{c}{a+b})-(a+b+c)=(a+b+c).1-(a+b+c)=(a+b+c)-(a+b+c)=0(dpcm)[/tex]