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ĐKXĐ: \(x,y\ne0\)
Hệ pt \(\Leftrightarrow\left\{{}\begin{matrix}\dfrac{6x+6y}{xy}=5\\\dfrac{4}{x}+\dfrac{3}{y}=1\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}\dfrac{6}{x}+\dfrac{6}{y}=5\\\dfrac{4}{x}+\dfrac{3}{y}=1\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}\dfrac{6}{x}+\dfrac{6}{y}=5\\\dfrac{3}{y}=1-\dfrac{4}{x}\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}\dfrac{6}{x}+2.\left(1-\dfrac{4}{x}\right)=5\\\dfrac{3}{y}=1-\dfrac{4}{x}\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}\dfrac{6}{x}+2-\dfrac{8}{x}=5\\\dfrac{3}{y}=1-\dfrac{4}{x}\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}\dfrac{-2}{x}=3\\\dfrac{3}{y}=1-\dfrac{4}{x}\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{-2}{3}\\y=\dfrac{3}{7}\end{matrix}\right.\)(tm)
Vậy...
\(\left\{{}\begin{matrix}6x+6y=5xy(1)\\\dfrac{4}{x}-\dfrac{3}{y}=1\end{matrix}\right.\)
Chia 2 vế cho xy thì (1)(vì `x,y ne 0`)
`<=>` $\begin{cases}\dfrac6x+\dfrac6y=5\\\dfrac{4}{x}-\dfrac{3}{y}=1\\\end{cases}$
`<=>` $\begin{cases}\dfrac6x+\dfrac6y=5\\\dfrac{8}{x}-\dfrac{6}{y}=2\\\end{cases}$
`<=>` $\begin{cases}\dfrac{14}{x}=7\\\dfrac6x+\dfrac6y=5\\\end{cases}$
`<=>` $\begin{cases}\dfrac{14}{x}=7\\\dfrac6x+\dfrac6y=5\\\end{cases}$
`<=>` $\begin{cases}x=2\\y=3\\\end{cases}$
Vậy HPT có nghiệm (x,y)=(2,3)
\(\left\{{}\begin{matrix}3x-3y=5\\5x+2y=23\end{matrix}\right.< =>\left\{{}\begin{matrix}6x-6y=10\\15x+6y=69\end{matrix}\right.< =>\left\{{}\begin{matrix}21x=79\\3x-3y=5\end{matrix}\right.\)
\(\left\{{}\begin{matrix}x=\dfrac{79}{21}\\y=\dfrac{44}{21}\end{matrix}\right.\)
vậy hệ pt có nghiệm (x,y)=(\(\dfrac{79}{21};\dfrac{44}{21}\))
6. \(\left\{{}\begin{matrix}2y-4=0\\3x+y=-4\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}y=2\\3x+2=-4\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}y=2\\x=-2\end{matrix}\right.\)
7. \(\left\{{}\begin{matrix}4x-6y=2\\x-\dfrac{3}{2}y=\dfrac{1}{2}\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{2+6y}{4}\\\dfrac{2+6y}{4}-\dfrac{3}{2}y=\dfrac{1}{2}\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{2+6y}{4}\\y=-2\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=-\dfrac{5}{2}\\y=-2\end{matrix}\right.\)
8. \(\left\{{}\begin{matrix}\dfrac{x}{3}+\dfrac{y}{2}=1\\2x+3y=\dfrac{2}{5}\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=\left(1-\dfrac{y}{2}\right).3\\6\left(1-\dfrac{y}{2}\right)+3y=\dfrac{2}{5}\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=3\left(1-\dfrac{y}{2}\right)\\y=\left(VNghiệm\right)\end{matrix}\right.\Leftrightarrow\) không tồn tại x, y
(Các câu khác tương tự nhé.)
\(\left\{{}\begin{matrix}\dfrac{2y+1}{18}=\dfrac{4y+1}{24}\\\dfrac{4y+1}{24}=\dfrac{6y+1}{6x}\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}4\left(2y+1\right)=3\left(4y+1\right)\\\dfrac{4y+1}{4}=\dfrac{6y+1}{x}\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}12y+3-8y-4=0\\\dfrac{4y+1}{4}=\dfrac{6y+1}{x}\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}y=\dfrac{1}{4}\\\dfrac{2}{4}=\dfrac{\dfrac{3}{2}+1}{x}\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}y=\dfrac{1}{4}\\\dfrac{5}{2}:x=\dfrac{1}{2}\end{matrix}\right.\Leftrightarrow\left(x,y\right)=\left(5;\dfrac{1}{4}\right)\)
Bài 2:
a: \(\Leftrightarrow\left\{{}\begin{matrix}2-x+y-3x-3y=5\\3x-3y+5x+5y=-2\end{matrix}\right.\)
=>-4x-2y=3 và 8x+2y=-2
=>x=1/4; y=-2
b: \(\Leftrightarrow\left\{{}\begin{matrix}\dfrac{5}{y-1}=1\\\dfrac{1}{x-2}+\dfrac{1}{y-1}=1\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}y-1=5\\\dfrac{1}{x-2}=1-\dfrac{1}{5}=\dfrac{4}{5}\end{matrix}\right.\)
=>y=6 và x-2=5/4
=>x=13/4; y=6
c: =>x+y=24 và 3x+y=78
=>-2x=-54 và x+y=24
=>x=27; y=-3
d: \(\Leftrightarrow\left\{{}\begin{matrix}2\sqrt{x-1}-6\sqrt{y+2}=4\\2\sqrt{x-1}+5\sqrt{y+2}=15\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}-11\sqrt{y+2}=-11\\\sqrt{x-1}=2+3\cdot1=5\end{matrix}\right.\)
=>y+2=1 và x-1=25
=>x=26; y=-1
Lời giải:
Phương hướng giải là bạn sử dụng phương pháp thế, biểu diễn $x$ theo $y$ qua 1 trong 2 PT, sau đó thế vô PT còn lại giải PT 1 ẩn $y$
a) \(\left\{\begin{matrix}
x-6y=17\\
5x+y=23\end{matrix}\right.\Leftrightarrow \left\{\begin{matrix}
x=17+6y\\
5x+y=23\end{matrix}\right.\)
\(\Rightarrow 5(17+6y)+y=23\)
\(\Leftrightarrow 31y=-62\Leftrightarrow y=-2\)
$x=17+6y=17+6(-2)=5$
Vậy $(x,y)=(5,-2)$
Các phần còn lại bạn giải tương tự
b) $(x,y)=(\frac{1}{4}, 0)$
c) $(x,y)=(3, 4)$
d) $(x,y)=(\frac{79}{21}, \frac{44}{21})$
ĐK: `x ne 2; y ne -1`
Đặt `{a=(1/(x-2)),(b=1/(y+1)):}`
Có: `{(2a+b=3),(4a-3b=1):}`
`<=>{(4a+2b=6),(4a-3b=1):}`
`<=>{(2a+b=3),(5b=5):}`
`<=>{(2a+1=3),(b=1):}`
`<=>{(a=1),(b=1):}`
``
`=>{(1/(x-2)=1),(1/(y+1)=1):}`
`<=>{(x-2=1),(y+1=1):}`
`<=>{(x=3),(y=0):}` (TM)
``
Vậy `(x;y)=(3;0)`.
Lời giải:
Ta có:
HPT \(\Leftrightarrow \left\{\begin{matrix} 6x+6y=5xy\\ 4y-3x=xy\end{matrix}\right.\)
\(\Rightarrow \left\{\begin{matrix} 6x+6y=5xy\\ 20y-15x=5xy\end{matrix}\right.\)
Lấy PT(1) - PT(2):
\(6x+6y-(20y-15x)=0\)
\(\Leftrightarrow 21x=14y\Leftrightarrow 3x=2y\Rightarrow y=1,5x\)
Thay vào PT ban đầu:
\(6x+6.1,5x=5x.1,5x\)
\(\Leftrightarrow 15x=7,5x^2\Leftrightarrow x(7,5x-15)=0\)
Vì $x\neq 0$ nên \(7,5x-15=0\Leftrightarrow x=2\Rightarrow y=1,5.2=3\)
Vậy $(x,y)=(2,3)$