Do \(x=2\) là nghiệm của phương trình nên:

\(\left\{{}\begin{matrix}2a+y=3\\2+ay=-1\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}y=3-2a\\ay=-3\end{matrix}\right.\)

\(\Rightarrow\left\{{}\begin{matrix}ay=3a-2a^2\\ay=-3\end{matrix}\right.\)

\(\Rightarrow3a-2a^2=-3\)

\(\Rightarrow2a^2-3a-3=0\Rightarrow a=\dfrac{3\pm\sqrt{33}}{4}\)

Thay vào ta được 

\(\left\{{}\begin{matrix}a=2a-1\\-1=a^2-2\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}a=1\\a^2-1=0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}a=1\\a=-1\end{matrix}\right.\)

Lời giải:
Cộng 2 pt theo vế có:

$3x=3m+3\Rightarrow x=m+1$

$y=x-(2m+1)=m+1-(2m+1)=-m$

Khi đó:
$(x+1)(y-3)<0$

$\Leftrightarrow (m+1+1)(-m-3)<0$

$\Leftrightarrow (m+2)(m+3)>0$

$\Leftrightarrow m>-2$ hoặc $m<-3$

a)

Khi m = 1, ta có:

{ x+2y=1+3   

  2x-3y=1

=> { x+2y=4

        2x-3y=1

=> { 2x+4y=8

        2x-3y=1

=> { x+2y=4

        2x-3y-2x-4y=1-8

=> { x=4-2y

       -7y = -7

=> { x = 2

        y = 1

Vậy khi m = 1 thì hệ phương trình có cặp nghệm

(x; y) = (2;1)

a) Thay m=1 vào HPT ta có: 

\(\left\{{}\begin{matrix}x+2y=4\\2x-3y=1\end{matrix}\right.\)

⇔\(\left\{{}\begin{matrix}2x+4y=8\\2x-3y=1\end{matrix}\right.\)

⇔\(\left\{{}\begin{matrix}2x+4y=8\\7y=7\end{matrix}\right.\)

⇔\(\left\{{}\begin{matrix}x=2\\y=1\end{matrix}\right.\)

Vậy HPT có nghiệm (x;y)= (2;1)

a) Thay m=1 vào hệ phương trình, ta được:

\(\left\{{}\begin{matrix}x+2y=4\\2x-3y=1\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}2x+4y=8\\2x-3y=1\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}7y=7\\x+2y=4\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}y=1\\x=4-2y=4-2=2\end{matrix}\right.\)

Vậy: Khi m=1 thì hệ phương trình có nghiệm duy nhất là (x,y)=(2;1)

b) Ta có: \(\left\{{}\begin{matrix}x+2y=m+3\\2x-3y=m\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}x=m+3-2y\\2\left(m+3-2y\right)-3y=m\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=m+3-2y\\2m+6-4y-3y-m=0\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}x=m+3-2y\\-7y+m+6=0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=m+3-2y\\-7y=-m-6\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}x=m+3-2y\\y=\dfrac{m+6}{7}\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=m+3-2\cdot\dfrac{m+6}{7}\\y=\dfrac{m+6}{7}\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}x=m+3-\dfrac{2m+12}{7}\\y=\dfrac{m+6}{7}\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{7m+21-2m-12}{7}=\dfrac{5m+9}{7}\\y=\dfrac{m+6}{7}\end{matrix}\right.\)

Để hệ phương trình có nghiệm duy nhất thỏa mãn x+y=3 thì \(\dfrac{5m+9}{7}+\dfrac{m+6}{7}=3\)

\(\Leftrightarrow6m+15=21\)

\(\Leftrightarrow6m=6\)

hay m=1

Vậy: Khi m=1 thì hệ phương trình có nghiệm duy nhất thỏa mãn x+y=3

a/ Thay  \(m=1\) vào hpt ta có :

\(\left\{{}\begin{matrix}x+2y=4\\2x-3y=1\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}x=2\\y=1\end{matrix}\right.\)

Vậy...

b/ Ta có :

\(\left\{{}\begin{matrix}x+2y=m+3\\2x-3y=m\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{m+3}{2y}\\\dfrac{2\left(m+3\right)}{2y}-3y=m\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{m+3}{2y}\\\dfrac{m+3}{y}-3y=m\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{m+3}{2y}\\m-3y^2+3=my\end{matrix}\right.\)

Thay x=1 và y=2 vào HPT, ta được:

\(\left\{{}\begin{matrix}a+2=3\\2+2=a^2+3\end{matrix}\right.\Leftrightarrow a=1\)

a, \(\left(I\right):\left\{{}\begin{matrix}2x+ay=b\\ax-by=1\end{matrix}\right.\)

Thay (x;y)=(1;-3) vào hpt có :

\(\left\{{}\begin{matrix}2-3a=b\\a+3b=1\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}3a+b=2\\a+3b=1\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}9a+3b=6\\a+3b=1\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}8a=5\\a+3b=1\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}a=\dfrac{5}{8}\\\dfrac{5}{8}+3b=1\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}a=\dfrac{5}{8}\\b=\dfrac{1}{8}\end{matrix}\right.\)

Vậy a=5/8 , b=1/8

Lời giải:

a)

Khi $m=1$ thì HPT trở thành:\(\left\{\begin{matrix} x-y=2\\ x+y=1\end{matrix}\right.\Rightarrow \left\{\begin{matrix} 2x=2+1\\ 2y=1-2\end{matrix}\right.\Leftrightarrow \left\{\begin{matrix} x=\frac{3}{2}\\ y=\frac{-1}{2}\end{matrix}\right.\)

b) 

HPT \(\Leftrightarrow \left\{\begin{matrix} mx-y=2\\ x=1-my\end{matrix}\right.\Rightarrow m(1-my)-y=2\)

\(\Leftrightarrow y(m^2+1)=m-2\Rightarrow y=\frac{m-2}{m^2+1}\)

\(x=1-my=1-\frac{m^2-2m}{m^2+1}=\frac{1+2m}{m^2+1}\)

Để $x+y=-1$

$\Leftrightarrow \frac{m-2}{m^2+1}+\frac{1+2m}{m^2+1}=-1$

$\Leftrightarrow \frac{3m-1}{m^2+1}=-1$

$\Rightarrow 3m-1=-m^2-1$

$\Leftrightarrow m^2+3m=0\Rightarrow m=0$ hoặc $m=-3$