\(1)-4x\left(x-5\right)-2x\left(8-2x\right)=-3\)
\(\Rightarrow-4x^2-\left(-20x\right)-16x+4x^2=-3\)
\(\Rightarrow20x-14x=-3\)
\(\Rightarrow6x=-3\)
\(\Rightarrow x=-\dfrac{1}{2}\)
Vậy \(x=-\dfrac{1}{2}\)
\(2)\) Theo bài ra, ta có: \(\dfrac{x^3}{8}=\dfrac{y^3}{64}=\dfrac{z^3}{216}\) và \(x^2+y^2+z^2=14\)
\(\Rightarrow\dfrac{x^3}{2^3}=\dfrac{y^3}{4^3}=\dfrac{z^3}{6^3}\)
\(\Rightarrow\left(\dfrac{x}{2}\right)^3=\left(\dfrac{y}{4}\right)^3=\left(\dfrac{z}{6}\right)^3\)
\(\Rightarrow\sqrt[3]{\left(\dfrac{x}{2}\right)^3}=\sqrt[3]{\left(\dfrac{y}{4}\right)^3}=\sqrt[3]{\left(\dfrac{z}{6}\right)^3}\)
\(\Rightarrow\dfrac{x}{2}=\dfrac{y}{4}=\dfrac{z}{6}\)
\(\Rightarrow\left(\dfrac{x}{2}\right)^2=\left(\dfrac{y}{4}\right)^2=\left(\dfrac{z}{6}\right)^2\)
\(\Rightarrow\dfrac{x^2}{2^2}=\dfrac{y^2}{4^2}=\dfrac{z^2}{6^2}\)
\(\Rightarrow\dfrac{x^2}{4}=\dfrac{y^2}{16}=\dfrac{z^2}{36}\)
Áp dụng tính chất của dãy tỉ số bằng nhau, ta có:
\(\dfrac{x^2}{4}=\dfrac{y^2}{16}=\dfrac{z^2}{36}=\dfrac{x^2+y^2+z^2}{4+16+36}=\dfrac{14}{56}=\dfrac{1}{4}\)
Suy ra:
\(+)\dfrac{x^2}{4}=\dfrac{1}{4}\Rightarrow x^2=\dfrac{1}{4}.4=1=\left(\pm1\right)^2\Rightarrow x=\pm1\)
\(+)\dfrac{y^2}{16}=\dfrac{1}{4}\Rightarrow y^2=\dfrac{1}{16}.4=\dfrac{1}{4}=\left(\pm\dfrac{1}{2}\right)^2\Rightarrow y=\pm\dfrac{1}{2}\)
\(+)\dfrac{z^2}{36}=\dfrac{1}{4}\Rightarrow z^2=\dfrac{1}{36}.4=\dfrac{1}{9}=\left(\pm\dfrac{1}{3}\right)^2\Rightarrow z=\pm\dfrac{1}{3}\)
Vậy \(\left(x;y;z\right)\in\left\{\left(-1;-\dfrac{1}{2};-\dfrac{1}{3}\right);\left(1;\dfrac{1}{2};\dfrac{1}{3}\right)\right\}\)
