a) Giải
Vì \(5x=2y\Rightarrow\dfrac{x}{2}=\dfrac{y}{5}\)
Đặt \(\dfrac{x}{2}=\dfrac{y}{5}=k\Rightarrow\left\{{}\begin{matrix}x=2k\\y=5k\end{matrix}\right.\)
Mà \(x-y=-7\)
\(\Rightarrow2k-5k=-7\)
\(\Rightarrow-3k=-7\)
\(\Rightarrow k=\dfrac{7}{3}\)
Vậy \(\left\{{}\begin{matrix}x=2k=2.\dfrac{7}{3}=\dfrac{14}{3}\\y=5k=5.\dfrac{7}{3}=\dfrac{35}{3}\end{matrix}\right.\)
b) Giải
Vì \(5x=7y\Rightarrow\dfrac{x}{7}=\dfrac{y}{5}\)
Đặt \(\dfrac{x}{7}=\dfrac{y}{5}=k\Rightarrow\left\{{}\begin{matrix}x=7k\\y=5k\end{matrix}\right.\)
Mà \(y-x=18\)
\(\Rightarrow5k-7k=18\)
\(\Rightarrow-2k=18\)
\(\Rightarrow k=-9\)
Vậy \(\left\{{}\begin{matrix}x=7k=7.\left(-9\right)=-63\\y=5k=5.\left(-9\right)=-45\end{matrix}\right.\)
c) Giải
Vì \(x:y=3:4\Rightarrow\dfrac{x}{3}=\dfrac{y}{4}\)
Đặt \(\dfrac{x}{3}=\dfrac{y}{4}=k\Rightarrow\left\{{}\begin{matrix}x=3k\\y=4k\end{matrix}\right.\)
Mà \(x+y=-21\)
\(\Rightarrow3k+4k=-21\)
\(\Rightarrow7k=-21\)
\(\Rightarrow k=-3\)
Vậy \(\left\{{}\begin{matrix}x=3k=3.\left(-3\right)=-9\\y=4k=4.\left(-3\right)=-12\end{matrix}\right.\)
d) Giải
Vì \(3x=7y\Rightarrow\dfrac{x}{7}=\dfrac{y}{3}\)
Đặt \(\dfrac{x}{7}=\dfrac{y}{3}=k\Rightarrow\left\{{}\begin{matrix}x=7k\\y=3k\end{matrix}\right.\)
Mà \(x-y=-16\)
\(\Rightarrow7k-3k=-16\)
\(\Rightarrow4k=-16\)
\(\Rightarrow k=-4\)
Vậy \(\left\{{}\begin{matrix}x=7k=7.\left(-4\right)=-28\\y=3k=3.\left(-4\right)=-12\end{matrix}\right.\)
y
a) ( -2xy2 )3.(-3xy) b) (-3xy2)2. 1 xy c) (-2x).(-0.5xyz)
Bài 7: Rút gọn rồi tính giá trị biểu thức A = 2x2y – 3xy2 – x2y + 2xy2 –xy + 1 tại x = -2; y = 1