2.
a. \(5x\left(x-1\right)=x-1\Leftrightarrow5x\left(x-1\right)-x+1=0\Leftrightarrow5\left(x-1\right)-\left(x-1\right)=0\Leftrightarrow\left(x-1\right)\left(5x-1\right)=0\Leftrightarrow\left[{}\begin{matrix}x-1=0\\5x-1=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=1\\5x=1\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=1\\x=\dfrac{1}{5}\end{matrix}\right.\)
Vậy \(x=1\) hoặc \(x=\dfrac{1}{5}\)
b. \(2\left(x+5\right)-x^{^2}-5x=0\Leftrightarrow2\left(x+5\right)-x\left(x+5\right)=0\Leftrightarrow\left(x+5\right)\left(2-x\right)=0\Leftrightarrow\left[{}\begin{matrix}x+5=0\\2-x=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=-5\\x=2\end{matrix}\right.\)
Vậy x=-5 hoặc x=2
c. thay \(a^3+b^3=\left(a+b\right)^3-3ab\left(a+b\right)\) ta có:
\(a^{^3}+b^{^3}+c^{^3}=3abc^{^3}\)
\(\Leftrightarrow a^3+b^3+c^3-3abc=0\)
\(\Leftrightarrow\left(a+b\right)^3-3ab\left(a+b\right)+c^3-3abc=0\)
\(\Leftrightarrow\left[\left(a+b\right)^3+c^{^3}\right]-3ab\left(a+b+c\right)=0\)
\(\Leftrightarrow\left(a+b+c\right)\left[\left(a+b\right)^{^2}-c\left(a+b\right)+c^{^2}\right]-3ab\left(a+b+c\right)=0\)
\(\Leftrightarrow\left(a+b+c\right)\left(a^{^2}+2ab+b^{^2}-ac-bc+c^{^2}-3ab\right)...\)
\(\Leftrightarrow\left(a+b+c\right)\left(a^{^2}+b^{^2}+c^{^2}-ab-ac-bc\right)=0\) luôn đúng vì a+b+c=0
\(\Rightarrow a^{^3}+b^{^3}+c^{^3}=3abc\left(đpcm\right)\)