a: Thay m=-2 vào hệ phương trình, ta được:

\(\left\{{}\begin{matrix}x-2y=-2+1=-1\\-2x+y=3\cdot\left(-2\right)-1=-7\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}2x-4y=-2\\-2x+y=-7\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}-3y=-9\\x-2y=-1\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}y=3\\x=2y-1=2\cdot3-1=5\end{matrix}\right.\)

b: Để hệ có nghiệm duy nhất thì \(\dfrac{1}{m}\ne\dfrac{m}{1}\)

=>\(m^2\ne1\)

=>\(m\notin\left\{1;-1\right\}\)

\(\left\{{}\begin{matrix}x+my=m+1\\mx+y=3m-1\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}x=m+1-my\\m\left(m+1-my\right)+y=3m-1\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}x=m+1-my\\m^2+m-m^2y+y=3m-1\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}x=m+1-my\\y\left(-m^2+1\right)=3m-1-m^2-m=-m^2+2m-1\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}x=m+1-my\\y\left(m-1\right)\left(m+1\right)=\left(m-1\right)^2\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}y=\dfrac{m-1}{m+1}\\x=m+1-m\cdot\dfrac{m-1}{m+1}=\left(m+1\right)-\dfrac{m^2-m}{m+1}\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}y=\dfrac{m-1}{m+1}\\x=\dfrac{m^2+2m+1-m^2+m}{m+1}=\dfrac{3m+1}{m+1}\end{matrix}\right.\)

\(x^2-y^2=4\)

=>\(\dfrac{\left(3m+1\right)^2-\left(m-1\right)^2}{\left(m+1\right)^2}=4\)

=>\(\dfrac{9m^2+6m+1-m^2+2m+1}{\left(m+1\right)^2}=4\)

=>\(8m^2+8m+2=4\left(m+1\right)^2\)

=>\(8m^2+8m+2-4m^2-8m-4=0\)

=>\(4m^2-2=0\)

=>\(m^2=\dfrac{1}{2}\)

=>\(m=\pm\dfrac{1}{\sqrt{2}}\)

a. Với `m=1`, ta có HPT: \(\left\{{}\begin{matrix}x+2y=18\\x-y=-6\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x-y=-6\\3y=24\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=2\\y=8\end{matrix}\right.\)

b. Theo đề bài `=>` \(\left\{{}\begin{matrix}mx+2y=18\\x-y=-6\\2x+y=9\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}mx+2y=18\\x=1\\y=7\end{matrix}\right.\)

`=> m=4`

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

\(\left\{{}\begin{matrix}x+2y=18\\x-y=-6\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}3y=24\\x-y=-6\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}y=8\\x=-6+y=-6+8=2\end{matrix}\right.\)

a. Bạn tự giải

b. Thế cặp nghiệm x=-1, y=3 vào hệ ban đầu ta được:

\(\left\{{}\begin{matrix}-1+3m=9\\-m-9=4\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}3m=10\\-m=13\end{matrix}\right.\)

\(\Rightarrow\) Không tồn tại m thỏa mãn

c. \(\Leftrightarrow\left\{{}\begin{matrix}mx+m^2y=9m\\mx-3y=4\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}\left(m^2+3\right)y=9m-4\\mx-3y=4\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}y=\dfrac{9m-4}{m^2+3}\\x=\dfrac{4m+27}{m^2+3}\end{matrix}\right.\)

Vậy với mọi m thì hệ luôn có nghiệm duy nhất như trên

a: Khi m=-1 thì hệ phương trình sẽ trở thành:

\(\left\{{}\begin{matrix}-x+y-3=3\\x-y-2\cdot\left(-1\right)+1=0\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}-x+y=6\\x-y=-3\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}0x=3\left(vôlý\right)\\x-y=-3\end{matrix}\right.\Leftrightarrow\left(x,y\right)\in\varnothing\)

b: \(\left\{{}\begin{matrix}mx+y-3=3\\x+my-2m+1=0\end{matrix}\right.\)(1)

=>\(\left\{{}\begin{matrix}mx+y=6\\x+my=2m-1\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}y=6-mx\\x+m\left(6-mx\right)=2m-1\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}x+6m-m^2x=2m-1\\y=6-mx\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}x\left(1-m^2\right)=-4m-1\\y=6-mx\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}x\left(m^2-1\right)=4m+1\\y=6-mx\end{matrix}\right.\)

TH1: m=1

Hệ phương trình (1) sẽ trở thành: 

\(\left\{{}\begin{matrix}x\cdot0=4\cdot1+1=5\\y=6-mx\end{matrix}\right.\Leftrightarrow\left(x,y\right)\in\varnothing\)

=>Loại

TH2: m=-1

Hệ phương trình (1) sẽ trở thành:

\(\left\{{}\begin{matrix}x\cdot0=-4+1=-3\\y=6-mx\end{matrix}\right.\Leftrightarrow\left(x,y\right)\in\varnothing\)

=>Loại

Th3: \(m\notin\left\{1;-1\right\}\)

Hệ phương trình (1) sẽ tương đương với \(\left\{{}\begin{matrix}x=\dfrac{4m+1}{m^2-1}\\y=6-mx=\dfrac{6\left(m^2-1\right)-m\left(4m+1\right)}{m^2-1}\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}x=\dfrac{4m+1}{m^2-1}\\y=\dfrac{6m^2-6-4m^2-m}{m^2-1}=\dfrac{2m^2-m-6}{m^2-1}\end{matrix}\right.\)

Để hệ có nghiệm duy nhất thì m/1<>1/m

=>\(m^2\ne1\)

=>\(m\notin\left\{1;-1\right\}\)

Để x nguyên thì \(4m+1⋮m^2-1\)

=>\(\left(4m+1\right)\left(4m-1\right)⋮m^2-1\)

=>\(16m^2-1⋮m^2-1\)

=>\(16m^2-16+15⋮m^2-1\)

=>\(m^2-1\inƯ\left(15\right)\)

=>\(m^2-1\in\left\{1;-1;3;-3;5;-5;15;-15\right\}\)

=>\(m^2\in\left\{2;0;4;6;16\right\}\)

=>\(m\in\left\{\sqrt{2};-\sqrt{2};0;2;-2;\sqrt{6};-\sqrt{6};4;-4\right\}\)

mà m nguyên

nên \(m\in\left\{0;2;4;-2;-4\right\}\left(2\right)\)

Để y nguyên thì \(2m^2-m-6⋮m^2-1\)

=>\(2m^2-2-m-4⋮m^2-1\)

=>\(m+4⋮m^2-1\)

=>\(\left(m+4\right)\left(m-4\right)⋮m^2-1\)

=>\(m^2-16⋮m^2-1\)

=>\(m^2-1-15⋮m^2-1\)

=>\(m^2-1\inƯ\left(-15\right)\)

=>\(m^2-1\in\left\{1;-1;3;-3;5;-5;15;-15\right\}\)

=>\(m^2\in\left\{2;0;4;6;16\right\}\)

=>\(m\in\left\{\sqrt{2};-\sqrt{2};0;2;-2;\sqrt{6};-\sqrt{6};4;-4\right\}\)

mà m nguyên

nên \(m\in\left\{0;2;4;-2;-4\right\}\left(3\right)\)

Từ (2),(3) suy ra \(m\in\left\{0;2;4;-2;-4\right\}\)

Thử lại, ta sẽ thấy m=4;m=-2 không thỏa mãn x nguyên; m=4;m=-2 không thỏa mãn y nguyên

=>\(m\in\left\{0;2;-4\right\}\)

a: Khi m=-1 thì hệ phương trình sẽ là:

\(\left\{{}\begin{matrix}x\cdot2\cdot\left(-1\right)-3y=2\cdot\left(-1\right)-1\\4x-\left(-1+5\right)y=2\end{matrix}\right.\)

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

=>\(\left\{{}\begin{matrix}-4x-6y=-6\\4x-4y=2\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}-6y-4y=-6+2\\x-y=\dfrac{1}{2}\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}-10y=-4\\x-y=\dfrac{1}{2}\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}y=\dfrac{2}{5}\\x=\dfrac{1}{2}+\dfrac{2}{5}=\dfrac{9}{10}\end{matrix}\right.\)

b: Để hệ phương trình có nghiệm duy nhất thì \(\dfrac{2m}{4}\ne-\dfrac{3}{-\left(m+5\right)}\)

=>\(\dfrac{m}{2}\ne\dfrac{3}{m+5}\)

=>\(m^2+5m\ne6\)

=>\(m^2+5m-6\ne0\)

=>\(\left(m+6\right)\left(m-1\right)\ne0\)

=>\(m\notin\left\{-6;1\right\}\)

c: \(\left\{{}\begin{matrix}2mx-3y=2m-1\\4x-\left(m+5\right)y=2\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}4mx-6y=4m-2\\4mx-\left(m^2+5m\right)y=2m\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}-6y+\left(m^2+5m\right)y=2m-2\\4x-\left(m+5\right)y=2\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}y\left(m^2+5m-6\right)=2m-2\\4x-\left(m+5\right)y=2\end{matrix}\right.\)(1)

Khi \(m\notin\left\{-6;1\right\}\) thì hệ phương trình (1) sẽ trở thành:

\(\left\{{}\begin{matrix}y=\dfrac{2m-2}{m^2+5m-6}=\dfrac{2\left(m-1\right)}{\left(m+6\right)\left(m-1\right)}=\dfrac{2}{m+6}\\4x=2+\left(m+5\right)y=2+\dfrac{2m+10}{m+6}=\dfrac{4m+22}{m+6}\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}y=\dfrac{2}{m+6}\\x=\dfrac{4m+22}{4m+24}=\dfrac{2m+11}{2m+12}\end{matrix}\right.\)

Để hệ có nghiệm duy nhất thỏa mãn x>0 và y>0 thì \(\left\{{}\begin{matrix}m\notin\left\{-6;1\right\}\\\dfrac{2}{m+6}>0\\\dfrac{2m+11}{2m+12}>0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}m\notin\left\{-6;1\right\}\\m+6>0\\\dfrac{2m+11}{m+6}>0\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}m\ne1\\m>-6\\\left[{}\begin{matrix}\left\{{}\begin{matrix}2m+11>0\\m+6>0\end{matrix}\right.\\\left\{{}\begin{matrix}2m+11< 0\\m+6< 0\end{matrix}\right.\end{matrix}\right.\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}m\ne1\\m>-6\\\left[{}\begin{matrix}m>-\dfrac{11}{2}\\m< -6\end{matrix}\right.\end{matrix}\right.\Leftrightarrow\)\(\left\{{}\begin{matrix}m\ne1\\m>-\dfrac{11}{2}\end{matrix}\right.\)

a: Khi m=-3 thì hệ phương trình sẽ là:

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

=>\(\left\{{}\begin{matrix}-3x+2y=1\\x+6y=-5\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}-3x+2y=1\\3x+18y=-15\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}20y=-14\\x+6y=-5\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}y=-\dfrac{7}{10}\\x=-5-6y=-5-6\cdot\dfrac{-7}{10}=\dfrac{42}{10}-5=-\dfrac{8}{10}=-\dfrac{4}{5}\end{matrix}\right.\)

b: \(\left\{{}\begin{matrix}mx+2y=1\\x-2my=m-2\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}x=2my+m-2\\m\left(2my+m-2\right)+2y=1\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}x=2my+m-2\\2m^2\cdot y+m^2-2m+2y=1\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}x=2my+m-2\\y\left(2m^2+2\right)=-m^2+2m+1\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}y=\dfrac{-m^2+2m+1}{2m^2+2}\\x=2m\cdot\dfrac{-m^2+2m+1}{2m^2+2}+m-2\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}y=\dfrac{-m^2+2m+1}{2m^2+2}\\x=\dfrac{m\left(-m^2+2m+1\right)}{m^2+1}+m-2\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}y=\dfrac{-m^2+2m+1}{2m^2+2}\\x=\dfrac{-m^3+2m^2+m+\left(m-2\right)\left(m^2+1\right)}{m^2+1}\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}x=\dfrac{-m^3+2m^2+m+m^3+m-2m^2-2}{m^2+1}=\dfrac{2m-2}{m^2+1}\\y=\dfrac{-m^2+2m+1}{2m^2+2}\end{matrix}\right.\)

x-2y=-1

=>\(\dfrac{2m-2}{m^2+1}-\dfrac{2\cdot\left(-m^2+2m+1\right)}{2m^2+2}=1\)

=>\(\dfrac{2m-2}{m^2+1}-\dfrac{-m^2+2m+1}{m^2+1}=1\)

=>\(\dfrac{2m-2+m^2-2m-1}{m^2+1}=1\)

=>\(m^2-3=m^2+1\)

=>-3=1(vô lý)

Xét hệ 

m x + y = 3 4 x + m y = 6 ⇔ y = 3 − m x 4 x + m 3 − m x = 6 ⇔ y = 3 − m x 4 x + 3 m − m 2 x = 6 ⇔ y = 3 − m x 4 − m 2 x = 6 − 3 m ⇔ y = 3 − m x                                 1 m 2 − 4 x = 3 m − 2       2

Hệ phương trình đã cho có nghiệm duy nhất ⇔ (2) có nghiệm duy nhất

m 2 – 4 ≠ 0 ⇔ m ≠ ± 2 ( * )

Khi đó hệ đã cho có nghiệm duy nhất

⇔ x = 3 m + 2 y = 3 − 3 m m + 2 ⇔ x = 3 m + 2 y = 6 m + 2

Ta có

x > 0 y > 2 ⇔ 3 m + 2 > 0 6 m + 2 > 1 ⇔ m + 2 > 0 4 − m m + 2 > 0 ⇔ m > − 2 4 − m > 0 ⇔ m > − 2 m < 4 ⇔ − 2 < m < 4

Kết hợp với (*) ta được giá trị m cần tìm là – 2 < m < 4; m ≠ 2

Đáp án: A

a) Với m = -2

=> hpt trở thành: \(\left\{{}\begin{matrix}x+y=2\\-2x-y=-2\end{matrix}\right.\) <=> \(\left\{{}\begin{matrix}y=2-x\\-x=0\end{matrix}\right.\) <=> \(\left\{{}\begin{matrix}x=0\\y=2\end{matrix}\right.\)

Vậy S = {0; 2}

b) Ta có: \(\left\{{}\begin{matrix}x+y=2\left(1\right)\\mx-y=m\left(2\right)\end{matrix}\right.\) 

=> x + mx = 2 + m 

<=> x(m + 1) = 2 + m

Để hpt có nghiệm duy nhất <=> \(m\ne-1\)

<=> x = \(\dfrac{m+2}{m+1}\) thay vào pt (1)

=> y = \(2-\dfrac{m+2}{m+1}=\dfrac{2m+2-m-2}{m+1}=\dfrac{m}{m+1}\)

Mà 3x - y = -10

=> \(3\cdot\dfrac{m+2}{m+1}-\dfrac{m}{m+1}=-10\)

<=> \(\dfrac{2m+6}{m+1}=-10\) <=> m + 3 = -5(m + 1)

<=> 6m = -8 

<=> m = -4/3

c) Để hpt có nghiệm <=> m \(\ne\)-1

Do x;y \(\in\) Z <=> \(\left\{{}\begin{matrix}\dfrac{m+2}{m+1}\in Z\\\dfrac{m}{m+1}\in Z\end{matrix}\right.\)

Ta có: \(x=\dfrac{m+2}{m+1}=1+\dfrac{1}{m+1}\)

Để x nguyên <=> 1 \(⋮\)m + 1

<=> m +1 \(\in\)Ư(1) = {1; -1}

<=> m \(\in\) {0; -2}

Thay vào y :

với m = 0 => y = \(\dfrac{0}{0+1}=0\)(tm)

m = -2 => y = \(\dfrac{-2}{-2+1}=2\)(tm)

Vậy ....