Question

Cho a,b,c khác 0 và phân biệt thỏa mãn a^3+b^3+c^3=3abc

Tính M=ab^2/a^2+b^2-c^2 +bc^2/b^2+c^2-a^2 + ca^2/c^2+a^2-b^2

Answer 1

mik ko biết

Answer 2

Ta có: a³+b³+c³=3abc

<=> (a+b+c)(a²+b²+c²-ab-bc-ca)=0

<=> (a+b+c)(2a²+2b²+2c²-2ab-2bc-2ca)=0

<=> (a+b+c)[(a-b)²+(b-c)²+(c-a)² ] = 0

<=> \(\orbr{\begin{cases}a+b+c=0\\\left(a-b\right)^2+\left(b-c\right)^2+\left(c-a\right)^2=0\end{cases}}\)

<=> \(\orbr{\begin{cases}a+b+c=0\\a=b=c\end{cases}}\)

Vì a,b,c phân biệt nên a+b+c=0 => \(\hept{\begin{cases}a=-\left(b+c\right)\\b=-\left(c+a\right)\\c=-\left(a+b\right)\end{cases}}\)(*)

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

Thay (*) vào M ta được:

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

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

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

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

Mà a+b+c=0

=> M=0

Vậy M=0

Answer 3

Sửa lại dòng (*)

Vì a,b,c phân nên a+b+c=0 => \(\hept{\begin{cases}a=-\left(b+c\right)\\b=-\left(a+c\right)\\c=-\left(a+b\right)\end{cases}\left(\text{*}\right)}\)