Question

[Bùi Thị Ngọc Anh]

cho a+b+c=0, a,b,c\(\ne\) 0.

Rút gọn:

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

Answer 1

[nguyen an]

a + b + c =0

⇒ -( a + b) = c

⇒ c² = (a+b)²

ta có \(\dfrac{ab}{a^{2^{ }}+b^{2^{ }}_{_{ }}-c^2}\)= \(\dfrac{ab}{a^{2^{ }}+b^{2^{ }}-\left(a+b\right)^2}\)=\(\dfrac{ab}{-2ab}\)=\(\dfrac{-1}{2}\)

tương tự với các số hạng còn lại kết quả là -3/2

Answer 2

[Nào Ai Biết]

Theo đề bài ta có :

a + b + c = 0

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

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

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

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

\(=\dfrac{ab}{-2ab}+\dfrac{bc}{-2bc}+\dfrac{ac}{-2ac}=-\dfrac{1}{2}-\dfrac{1}{2}-\dfrac{1}{2}=-\dfrac{3}{2}\)