Ta có: a+b+c=0\(\Leftrightarrow\)b+c=-a

Bình phương hai vế có: (b+c)²=a²

⇔ b²+2bc+c²=a²\(\Leftrightarrow\) b²+c²-a²=-2bc

Tương tự, ta có: c²+a²-b²=-2ca

                           a²+b²-c²=-2ab

→ A=\(-\dfrac{1}{2bc}-\dfrac{1}{2ca}-\dfrac{1}{2ab}=\dfrac{-\left(a+b+c\right)}{2abc}=0\)(vì a+b+c=0)

Vậy A=0

\(1,a+b+c=0\Leftrightarrow a=-b-c\Leftrightarrow a^2=b^2+2bc+c^2\Leftrightarrow b^2+c^2=a^2-2bc\)

Tương tự: \(\left\{{}\begin{matrix}a^2+b^2=c^2-2ab\\c^2+a^2=b^2-2ac\end{matrix}\right.\)

\(\Leftrightarrow N=\dfrac{a^2}{a^2-a^2+2bc}+\dfrac{b^2}{b^2-b^2+2ca}+\dfrac{c^2}{c^2-c^2+2ac}\\ \Leftrightarrow N=\dfrac{a^2}{2bc}+\dfrac{b^2}{2ac}+\dfrac{c^2}{2bc}=\dfrac{a^3+b^3+c^3}{2abc}=\dfrac{a^3+b^3+c^3-3abc+3abc}{2abc}\\ \Leftrightarrow N=\dfrac{\left(a+b+c\right)\left(a^2+b^2+c^2-ab-bc-ca\right)+3abc}{2abc}\\ \Leftrightarrow N=\dfrac{3abc}{2abc}=\dfrac{3}{2}\)

\(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}=0\Leftrightarrow ab+bc+ca=0\Rightarrow\left\{{}\begin{matrix}bc=-ab-ac\\ab=-bc-ac\\ac=-ab-bc\end{matrix}\right.\)

\(M=\dfrac{1}{a^2+bc-ab-ac}+\dfrac{1}{b^2+ac-ab-bc}+\dfrac{1}{c^2+ab-bc-ac}\)

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

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

\(=\dfrac{b-c-\left(a-c\right)+a-b}{\left(a-b\right)\left(b-c\right)\left(a-c\right)}=0\)

Lời giải:

Từ \(a+b+c=0\Rightarrow a=-(b+c)\)

\(\Rightarrow a^2=[-(b+c)]^2=b^2+2bc+c^2\)

\(\Rightarrow b^2+c^2-a^2=b^2+c^2-(b^2+2bc+c^2)=-2bc\)

\(\Rightarrow \frac{1}{b^2+c^2-a^2}=\frac{1}{-2bc}=\frac{-a}{2abc}\)

Hoàn toàn tương tự với các biểu thức còn lại và cộng theo vế:

\(A=\frac{-a}{2abc}+\frac{-b}{2abc}+\frac{-c}{2abc}=\frac{-(a+b+c)}{2abc}=0\)

ta có

a+b+c =0

<=> a+b=-c

<=>(a+b)² =(-c)²

<=>a²+b²+2ab=c²

<=>a²+b²-c²=-2ab

tương tự ta đc

c²+a²-b²=-2ac

b²+c²-a²=-2bc

thay vào A ta có

\(A=\dfrac{-1}{2bc}-\dfrac{1}{2ac}-\dfrac{1}{2ab}\)

<=> A=\(\dfrac{-a}{2abc}-\dfrac{b}{2abc}-\dfrac{c}{2abc}\)

<=> A=\(\dfrac{-\left(a+b+c\right)}{2abc}=0\) (vì a+b+c=0)

\(\dfrac{a^2}{a^2-b^2-c^2}=\dfrac{a^2}{\left(a-b\right)\left(a+b\right)-c^2}=\dfrac{a^2}{\left(a-b\right)\left(-c\right)-c^2}=\dfrac{a^2}{c\left(b-a-c\right)}=\dfrac{a^2}{2bc}\\ \Leftrightarrow M=\sum\dfrac{a^2}{a^2-b^2-c^2}=\sum\dfrac{a^2}{2bc}=\dfrac{a^3+b^3+c^3}{2abc}\\ \Leftrightarrow M=\dfrac{\left(a+b+c\right)\left(a^2+b^2+c^2-ab-bc-ca\right)}{2abc}=0\)

Lời giải:

\(a=\frac{1}{x}, b=\frac{1}{y}, c=\frac{1}{z}\)

\(A=\frac{\frac{1}{x^2y^2}}{(\frac{1}{x^3}+\frac{1}{y^3}).\frac{1}{z^2}}=\frac{z^2}{x^2y^2.\frac{x^3+y^3}{x^3y^3}}=\frac{z^2}{\frac{x^3+y^3}{xy}}=\frac{xyz^2}{x^3+y^3}\)

\(a+b=-c\Leftrightarrow\left(a+b\right)^3=-c^3\)

\(\Leftrightarrow a^3+b^3+3ab\left(a+b\right)=-c^3\)

\(\Leftrightarrow a^3+b^3+c^3=-3ab\left(a+b\right)=3abc\)

\(A=\dfrac{a^3+b^3+c^3}{abc}=\dfrac{3abc}{abc}=3\)

Bài 2:

a) \(A=\dfrac{a^2}{bc}+\dfrac{b^2}{ca}+\dfrac{c^2}{ab}\)

\(A=\dfrac{a^3}{abc}+\dfrac{b^3}{abc}+\dfrac{c^3}{abc}\)

\(A=\dfrac{1}{abc}\left(a^3+b^3+c^3\right)\)

\(A=\dfrac{1}{abc}\left[\left(a+b\right)^3-3ab\left(a+b\right)+c^3\right]\)

Vì \(a+b+c=0\)

Nên a + b = -c (1)

Thay (1) vào A, ta được:

\(A=\dfrac{1}{abc}\left[\left(-c\right)^3-3ab\left(-c\right)+c^3\right]\)

\(A=\dfrac{1}{abc}.3abc\)

\(A=3\)

b) \(B=\dfrac{a^2}{a^2-b^2-c^2}+\dfrac{b^2}{b^2-c^2-a^2}+\dfrac{c^2}{c^2-a^2-b^2}\)

\(B=\dfrac{a^2}{a^2-\left(b^2+c^2\right)}+\dfrac{b^2}{b^2-\left(c^2+a^2\right)}+\dfrac{c^2}{c^2-\left(a^2+b^2\right)}\)

Vì \(a+b+c=0\)

Nên b + c = -a

=> ( b + c )² = (-a)²

=> b² + c² + 2bc = a²

=> b² + c² = a² - 2bc (1)

Tương tự ta có: c² + a² = b² - 2ac (2)

a² + b² = c - 2ab (3)

Thay (1), (2) và (3) vào B, ta được:

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

\(B=\dfrac{a^2}{a^2-a^2+2bc}+\dfrac{b^2}{b^2-b^2+2ac}+\dfrac{c^2}{c^2-c^2+2ab}\)

\(B=\dfrac{a^2}{2bc}+\dfrac{b^2}{2ac}+\dfrac{c^2}{2ab}\)

\(B=\dfrac{a^3}{2abc}+\dfrac{b^3}{2abc}+\dfrac{c^3}{2abc}\)

\(B=\dfrac{1}{2abc}\left(a^3+b^3+c^3\right)\)

Mà \(a^3+b^3+c^3=3abc\) ( câu a )

\(\Rightarrow B=\dfrac{1}{2abc}.3abc\)

\(\Rightarrow B=\dfrac{3}{2}\)

Bài 1:

a) GT: abc = 2

\(M=\dfrac{a}{ab+a+2}+\dfrac{b}{bc+b+1}+\dfrac{2c}{ac+2c+2}\)

\(M=\dfrac{a}{ab+a+abc}+\dfrac{b}{bc+b+1}+\dfrac{2cb}{abc+2cb+2b}\)

\(M=\dfrac{a}{a\left(b+1+bc\right)}+\dfrac{b}{bc+b+1}+\dfrac{2cb}{2+2cb+2b}\)

\(M=\dfrac{1}{bc+b+1}+\dfrac{b}{bc+b+1}+\dfrac{2cb}{2\left(1+cb+b\right)}\)

\(M=\dfrac{1}{bc+b+1}+\dfrac{b}{bc+b+1}+\dfrac{bc}{bc+b+1}\)

\(M=\dfrac{1+b+bc}{bc+b+1}\)

\(M=1\)

b) GT: abc = 1

\(N=\dfrac{a}{ab+a+1}+\dfrac{b}{bc+b+1}+\dfrac{c}{ac+c+1}\)

\(N=\dfrac{a}{ab+a+abc}+\dfrac{b}{bc+b+1}+\dfrac{cb}{b\left(ac+c+1\right)}\)

\(N=\dfrac{a}{a\left(b+1+bc\right)}+\dfrac{b}{bc+b+1}+\dfrac{bc}{abc+bc+b}\)

\(N=\dfrac{1}{bc+b+1}+\dfrac{b}{bc+b+1}+\dfrac{bc}{bc+b+1}\)

\(N=\dfrac{1+b+bc}{bc+b+1}\)

\(N=1\)