Ta có : \(\left(a+b+c\right)^2=a^2+b^2+c^2+2\left(ab+bc+ac\right)=a^2+b^2+c^2\)

\(\Leftrightarrow2\left(ab+ac+bc\right)=0\Rightarrow ab+ac+bc=0\Rightarrow\hept{\begin{cases}ab=-ac-bc\\ac=-ab-bc\\bc=-ac-ab\end{cases}}\)

Nên \(\frac{a^2}{a^2+2bc}=\frac{a^2+ab+bc+ac}{a^2+bc-ac-ab}=\frac{\left(a+c\right)\left(a+b\right)}{\left(a-c\right)\left(a-b\right)}\)

\(\frac{b^2}{b^2+2ac}=\frac{b^2+ab+bc+ac}{b^2+ac-ab-bc}=\frac{\left(a+b\right)\left(b+c\right)}{\left(b-a\right)\left(b-c\right)}\)

\(\frac{c^2}{b^2+2ab}=\frac{c^2+ab+ac+bc}{b^2+ab-ac-bc}=\frac{\left(c+b\right)\left(c+a\right)}{\left(c-b\right)\left(c-a\right)}\)

\(P=\frac{\left(a+b\right)\left(a+c\right)}{\left(a-b\right)\left(a-c\right)}+\frac{\left(a+b\right)\left(b+c\right)}{\left(b-a\right)\left(b-c\right)}+\frac{\left(c+b\right)\left(c+a\right)}{\left(c-b\right)\left(c-a\right)}\)

\(=\frac{\left(a+b\right)\left(a+c\right)\left(b-c\right)+\left(a+b\right)\left(b+c\right)\left(c-a\right)+\left(c+b\right)\left(c+a\right)\left(a-b\right)}{\left(a-b\right)\left(a-c\right)\left(b-c\right)}\)

\(=\frac{\left(a+b\right)\left[\left(a+c\right)\left(b-c\right)+\left(b+c\right)\left(c-a\right)\right]+\left(c+b\right)\left(c+a\right)\left(a-b\right)}{\left(a-b\right)\left(a-c\right)\left(b-c\right)}\)

\(=\frac{\left(a+b\right)\left(2bc-2ac\right)+\left(c+b\right)\left(c+a\right)\left(a-b\right)}{\left(a-b\right)\left(a-c\right)\left(b-c\right)}\)

\(=\frac{-2c\left(a+b\right)\left(a-b\right)+\left(c+b\right)\left(c+a\right)\left(a-b\right)}{\left(a-b\right)\left(a-c\right)\left(b-c\right)}\)

\(=\frac{\left(a-b\right)\left[-2c\left(a+b\right)+\left(b+c\right)\left(c+a\right)\right]}{\left(a-b\right)\left(a-c\right)\left(b-c\right)}\)

\(=\frac{\left(a-b\right)\left(-a^2+ab+c^2-bc\right)}{\left(a-b\right)\left(a-c\right)\left(b-c\right)}\)

\(=\frac{\left(a-b\right)\left(a-c\right)\left(b-c\right)}{\left(a-b\right)\left(a-c\right)\left(b-c\right)}=1\)

Vậy \(P=1\)

cuối cùng P bằng 1 yên tâm mình tính rùi

Nhân khai triển tử và mẫu của B, thấy ab + bc + ca thì thay bằng 1

ta có : a+b+c=0=>a+b=-c ; b+c=-a ; a+c=-b 

ta có: M= \(\frac{2ab}{a^2+\left(b+c\right)\left(b-c\right)}+\frac{2bc}{b^2+\left(c+a\right)\left(c-a\right)}+\frac{2ca}{c^2+\left(a+b\right)\left(a-b\right)}\)

M=\(\frac{2ab}{a^2-a\left(b-c\right)}+\frac{2bc}{b^2-b\left(c-a\right)}+\frac{2ca}{c^2-c\left(a-b\right)}\)

M=\(\frac{2ab}{a\left(a-b+c\right)}+\frac{2bc}{b\left(b-c+a\right)}+\frac{2ca}{c\left(c-a+b\right)}\)

M=\(\frac{2ab}{-ab+\left(a+c\right)}+\frac{2bc}{-bc+\left(a+b\right)}+\frac{2ac}{-ac+\left(b+c\right)}\)

M=\(\frac{2ab}{-2ab}+\frac{2bc}{-2bc}+\frac{2ca}{-2ca}\)

M=-1-1-1=-3

Vậy với a+b+c=0 thì M=-3

Bài giải

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a) Ta có : \(a^2+1=a^2+ab+bc+ac=a\left(a+b\right)+c\left(a+b\right)=\left(a+b\right)\left(a+c\right)\)

Tương tự : \(b^2+1=\left(b+a\right)\left(b+c\right)\) ; \(c^2+1=\left(c+a\right)\left(c+b\right)\)

Suy ra \(\left(1+a^2\right)\left(1+b^2\right)\left(1+c^2\right)=\left(a+b\right)^2\left(b+c\right)^2\left(c+a\right)^2\)

Vậy \(A=\frac{\left(a+b\right)^2\left(b+c\right)^2\left(c+a\right)^2}{\left(a+b\right)^2\left(b+c\right)^2\left(c+a\right)^2}=1\)

b) Ta có ; \(a^2+2bc-1=a^2+2bc-\left(ab+bc+ac\right)=a^2-ab+bc-ac=a\left(a-b\right)-c\left(a-b\right)\)

\(=\left(a-b\right)\left(a-c\right)\)

Tương tự : \(b^2+2ac-1=\left(a-b\right)\left(c-b\right)\) ; \(c^2+2ab-1=\left(a-c\right)\left(b-c\right)\)

Suy ra \(\left(a^2+2bc-1\right)\left(b^2+2ac-1\right)\left(c^2+2ab-1\right)=\left(a-b\right)^2.\left(c-a\right)^2.\left[-\left(b-c\right)^2\right]\)

Vậy : \(B=\frac{-\left(a-b\right)^2\left(b-c\right)^2\left(c-a\right)^2}{\left(a-b\right)^2\left(b-c\right)^2\left(c-a\right)}=-1\)