Question

[cười ha hả bà]

  giúp e với ạ,  e cảm ơn

Answer 1

[Nguyễn Lê Phước Thịnh]

Bài 8:

a: \(\dfrac{x}{5}=\dfrac{y}{6}\)

=>\(\dfrac{x}{20}=\dfrac{y}{24}\)

\(\dfrac{y}{8}=\dfrac{z}{7}\)

=>\(\dfrac{y}{24}=\dfrac{z}{21}\)

Do đó: \(\dfrac{x}{20}=\dfrac{y}{24}=\dfrac{z}{21}=k\)

=>x=20k;y=24k;z=21k

x+y-z=69

=>20k+24k-21k=69

=>23k=69

=>k=3

=>\(x=20\cdot3=60;y=24\cdot3=72;z=21\cdot3=63\)

b: Đặt \(\dfrac{x}{3}=\dfrac{y}{4}=\dfrac{z}{5}=k\)

=>x=3k;y=4k;z=5k

\(2x^2+2y^2-3z^2=-100\)

=>\(2\cdot\left(3k\right)^2+2\cdot\left(4k\right)^2-3\cdot\left(5k\right)^2=-100\)

=>\(k^2=4\)

=>\(\left[{}\begin{matrix}k=2\\k=-2\end{matrix}\right.\)

TH1: k=2

=>\(x=3\cdot2=6;y=4\cdot2=8;z=5\cdot2=10\)

TH2: k=-2

=>\(x=3\cdot\left(-2\right)=-6;y=4\cdot\left(-2\right)=-8;z=5\cdot\left(-2\right)=-10\)