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a/ \(\Leftrightarrow\left\{{}\begin{matrix}4x^2-16xy+4y^2=4\\y^2-3xy=4\end{matrix}\right.\)
\(\Rightarrow4x^2-13xy+3y^2=0\)
\(\Leftrightarrow\left(x-3y\right)\left(4x-y\right)=0\)
\(\Rightarrow\left[{}\begin{matrix}x=3y\\y=4x\end{matrix}\right.\)
Thay vào pt sau: \(\left[{}\begin{matrix}y^2-3y.y=4\left(vn\right)\\\left(4x\right)^2-3x.4x=4\end{matrix}\right.\)
\(\Rightarrow x^2=1\Rightarrow\left[{}\begin{matrix}x=1;y=4\\x=-1;y=-4\end{matrix}\right.\)
b/
\(\Leftrightarrow\left\{{}\begin{matrix}4x^2-6xy+2y^2=6\\x^2+2xy-2y^2=6\end{matrix}\right.\)
\(\Rightarrow3x^2-8xy+4y^2=0\)
\(\Leftrightarrow\left(x-2y\right)\left(3x-2y\right)=0\Rightarrow\left[{}\begin{matrix}x=2y\\x=\frac{2}{3}y\end{matrix}\right.\)
Thay vào pt đầu: \(\left[{}\begin{matrix}2\left(2y\right)^2-3.2y.y+y^2=3\\2\left(\frac{2}{3}y\right)^2-3.\frac{2}{3}y.y+y^2=3\end{matrix}\right.\) bạn tự giải nốt
1,ĐK: \(x,y\ne-2\)
HPT<=> \(\left\{{}\begin{matrix}x\left(x+2\right)+y\left(y+2\right)=\left(x+2\right)\left(y+2\right)\left(1\right)\\x^2\left(x+2\right)^2+y^2\left(y+2\right)^2=\left(x+2\right)^2\left(y+2\right)^2\end{matrix}\right.\)
<=> \(\left\{{}\begin{matrix}x^2\left(x+2\right)^2+2xy\left(x+2\right)\left(y+2\right)+y^2\left(y+2\right)^2=\left(x+2\right)^2\left(y+2\right)^2\\x^2\left(x+2\right)^2+y^2\left(y+2\right)^2=\left(x+2\right)^2\left(y+2\right)^2\end{matrix}\right.\)
=> \(2xy\left(x+2\right)\left(y+2\right)=0\)
<=>\(2xy=0\) (do x+2 và y+2 \(\ne0\))
<=> \(\left[{}\begin{matrix}x=0\\y=0\end{matrix}\right.\)
Tại x=0 thay vào (1) có: \(y\left(y+2\right)=2\left(y+2\right)\) <=> y= \(\pm2\) => y=2 (vì y khác -2)
Tại y=0 thay vào (1) có: \(x\left(x+2\right)=2\left(x+2\right)\) => x=2
Vậy HPT có 2 nghiệm duy nhất (2,0),(0,2)
2, ĐK: \(y\ne-1\)
HPT <=> \(\left\{{}\begin{matrix}x^2=2\left(x+3\right)\left(y+1\right)\left(1\right)\\\frac{3x^2}{y+1}=4-x\end{matrix}\right.\)
=> \(\frac{6\left(3+x\right)\left(y+1\right)}{y+1}=4-x\)
<=> 6(x+3)=4-x
<=> \(14=-7x\)
<=> \(x=-2\) thay vào (1) có \(4=2\left(y+1\right)\)
<=>y=1\(\)( tm)
Vậy hpt có một nghiệm duy nhất (-2,1)
3,\(\left\{{}\begin{matrix}x^2-y=y^2-x\left(1\right)\\x^2-x=y+3\left(2\right)\end{matrix}\right.\)
PT (1) <=> \(\left(x-y\right)\left(x+y\right)+\left(x-y\right)=0\)
<=> (x-y)(x+y+1)=0
<=>\(\left[{}\begin{matrix}x=y\\y=-x-1\end{matrix}\right.\)
Tại x=y thay vào (2) có \(y^2-y=y+3\) <=> \(y^2-2y-3=0\) <=> (y-3)(y+1)=0 <=> \(\left[{}\begin{matrix}y=3\\y=-1\end{matrix}\right.\) => \(\left[{}\begin{matrix}x=3\\x=-1\end{matrix}\right.\)
Tại y=-1-x thay vào (2) có: \(x^2-x=-1-x+3\) <=> \(x^2=2\) <=> \(\left[{}\begin{matrix}x=\sqrt{2}\\x=-\sqrt{2}\end{matrix}\right.\) => \(\left[{}\begin{matrix}y=-1-\sqrt{2}\\y=-1+\sqrt{2}\end{matrix}\right.\)
Vậy hpt có 4 nghiệm (3,3),(-1,-1), ( \(\sqrt{2},-1-\sqrt{2}\)),( \(-\sqrt{2},-1+\sqrt{2}\))
4,\(\left\{{}\begin{matrix}x+y+\frac{1}{x}+\frac{1}{y}=\frac{9}{2}\left(1\right)\\xy+\frac{1}{xy}+\frac{x}{y}+\frac{y}{x}=5\left(2\right)\end{matrix}\right.\)(đk:\(x\ne0,y\ne0\))
<=> \(\left\{{}\begin{matrix}\left(x+\frac{1}{x}\right)+\left(y+\frac{1}{y}\right)=\frac{9}{2}\\\left(y+\frac{1}{y}\right)\left(x+\frac{1}{x}\right)=5\end{matrix}\right.\)
Đặt \(\left\{{}\begin{matrix}x+\frac{1}{x}=u\\y+\frac{1}{y}=v\end{matrix}\right.\)
Có \(\left\{{}\begin{matrix}u+v=\frac{9}{2}\\uv=5\end{matrix}\right.\) <=> \(\left\{{}\begin{matrix}u=\frac{9}{2}-v\\v\left(\frac{9}{2}-v\right)=5\end{matrix}\right.\) <=> \(\left\{{}\begin{matrix}u=\frac{9}{2}-v\\\left(v-\frac{5}{2}\right)\left(v-2\right)=0\end{matrix}\right.\) <=> \(\left\{{}\begin{matrix}u=\frac{9}{2}-v\\\left[{}\begin{matrix}v=\frac{5}{2}\\v=2\end{matrix}\right.\end{matrix}\right.\) <=> \(\left\{{}\begin{matrix}\left[{}\begin{matrix}v=\frac{5}{2}\\u=2\end{matrix}\right.\\\left[{}\begin{matrix}v=2\\u=\frac{5}{2}\end{matrix}\right.\end{matrix}\right.\)
Tại \(\left\{{}\begin{matrix}v=\frac{5}{2}\\u=2\end{matrix}\right.\) <=> \(\left\{{}\begin{matrix}x+\frac{1}{x}=2\\y+\frac{1}{y}=\frac{5}{2}\end{matrix}\right.\)
<=> \(\left\{{}\begin{matrix}\left(x-1\right)^2=0\\\left(y-2\right)\left(y-\frac{1}{2}\right)=0\end{matrix}\right.\) <=> \(\left\{{}\begin{matrix}x=1\\\left[{}\begin{matrix}y=2\\y=\frac{1}{2}\end{matrix}\right.\end{matrix}\right.\) <=> \(\left\{{}\begin{matrix}\left[{}\begin{matrix}x=1\\y=2\end{matrix}\right.\\\left[{}\begin{matrix}x=1\\y=\frac{1}{2}\end{matrix}\right.\end{matrix}\right.\)
Tại \(\left\{{}\begin{matrix}v=2\\u=\frac{5}{2}\end{matrix}\right.\) <=> \(\left\{{}\begin{matrix}x+\frac{1}{x}=\frac{5}{2}\\y+\frac{1}{y}=2\end{matrix}\right.\)
<=> \(\left\{{}\begin{matrix}\left(x-2\right)\left(x-\frac{1}{2}\right)=0\\\left(y-1\right)^2=0\end{matrix}\right.\) <=> \(\left\{{}\begin{matrix}\left[{}\begin{matrix}x=2\\x=\frac{1}{2}\end{matrix}\right.\\y=1\end{matrix}\right.\) <=> \(\left\{{}\begin{matrix}\left[{}\begin{matrix}x=2\\y=1\end{matrix}\right.\\\left[{}\begin{matrix}x=\frac{1}{2}\\y=1\end{matrix}\right.\end{matrix}\right.\)
Vậy hpt có 4 nghiệm (1,2),( \(1,\frac{1}{2}\)) ,( 2,1),(\(\frac{1}{2},1\)).
10.
\(\left\{{}\begin{matrix}2x^2-3xy+y^2+x-y=0\\x^2+x+1=y^2\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}2x^2-2xy-xy+y^2+x-y=0\\x^2+x+1=y^2\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}\left(x-y\right)\left(2x-y+1\right)=0\\x^2+x+1=y^2\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}\left[{}\begin{matrix}x=y\\y=2x+1\end{matrix}\right.\\x^2+x+1=y^2\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}x=y\\x^2+x+1=y^2\end{matrix}\right.\\\left\{{}\begin{matrix}y=2x+1\\x^2+x+1=y^2\end{matrix}\right.\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}x=y\\x^2+x+1=x^2\end{matrix}\right.\\\left\{{}\begin{matrix}y=2x+1\\x^2+x+1=\left(2x+1\right)^2\end{matrix}\right.\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}x=y\\x=-1\end{matrix}\right.\\\left\{{}\begin{matrix}y=2x+1\\3x\left(x+1\right)=0\end{matrix}\right.\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=y=1\\\left[{}\begin{matrix}\left\{{}\begin{matrix}y=2x+1\\x=0\end{matrix}\right.\\\left\{{}\begin{matrix}y=2x+1\\x=-1\end{matrix}\right.\end{matrix}\right.\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=y=-1\\\left\{{}\begin{matrix}x=0\\y=-\frac{1}{2}\end{matrix}\right.\\\left\{{}\begin{matrix}x=-1\\y=-1\end{matrix}\right.\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=y=-1\\\left\{{}\begin{matrix}x=0\\y=-\frac{1}{2}\end{matrix}\right.\end{matrix}\right.\)
\(\Leftrightarrow xy\left(x+y\right)+\left(x+y\right)^2-2xy=4\)
\(\Leftrightarrow xy\left(x+y-2\right)+\left(x+y-2\right)\left(x+y+2\right)=0\)
\(\Leftrightarrow\left(x+y-2\right)\left(x+y+xy+2\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x+y-2=0\left(1\right)\\x+y+xy+2=0\left(2\right)\end{matrix}\right.\)
Xét (1) \(\Leftrightarrow y=2-x\) thay vào pt đầu: ....
Xét (2): kết hợp với pt đầu ta được:
\(\left\{{}\begin{matrix}x+y+xy+2=0\\\left(x+y\right)^3-3xy\left(x+y\right)-3xy=-1\end{matrix}\right.\)
Đặt \(\left\{{}\begin{matrix}x+y=a\\xy=b\end{matrix}\right.\) với \(a^2\ge4b\)
\(\Rightarrow\left\{{}\begin{matrix}a+b+2=0\\a^3-3ab-3b=-1\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}a+b+2=0\\\left(a+1\right)\left(a^2-a+1\right)-3b\left(a+1\right)=0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}a+b+2=0\\\left(a+1\right)\left(a^2-a+1-3b\right)=0\end{matrix}\right.\)
\(\Leftrightarrow...\)
\(x^3+y^3+3xy=1\Leftrightarrow\left(x+y\right)^3-1-3xy\left(x+y\right)+3xy=0\)
\(\Leftrightarrow\left(x+y-1\right)\left[\left(x+y\right)^2+x+y+1\right]-3xy\left(x+y-1\right)=0\)
\(\Leftrightarrow\left(x+y-1\right)\left(x^2+y^2-xy+x+y+1\right)=0\)
\(\Leftrightarrow\left(x+y-1\right)\left[\left(x-y\right)^2+\left(x+1\right)^2+\left(y+1\right)^2\right]=0\)
\(\Rightarrow\left[{}\begin{matrix}x+y-1=0\\x=y=-1\end{matrix}\right.\)
TH1: \(x=y=-1\) thế vào pt dưới kiểm tra ko thỏa mãn
TH2: \(y=1-x\) thế vào pt dưới:
\(\sqrt{\left(4-x\right)\left(x+12\right)}=\dfrac{27}{x+3}\) (ĐKXĐ: \(-12\le x\le4;x\ne-3\))
- Với \(x< -3\) pt vô nghiệm, với \(x>-3\)
Đặt \(x+3=t>0\)
\(\Rightarrow\sqrt{\left(t+9\right)\left(7-t\right)}=\dfrac{27}{t}\Leftrightarrow64-\left(t+1\right)^2=\dfrac{27^2}{t^2}\)
\(\Leftrightarrow64=\dfrac{27^2}{t^2}+\left(t+1\right)^2=\dfrac{25^2}{t^2}+t^2+\dfrac{104}{t^2}+t+t+1\ge2\sqrt{\dfrac{25^2t^2}{t^2}}+3\sqrt[3]{\dfrac{104t^2}{t^2}}+1>65\) (vô lý)
Vậy hệ vô nghiệm
(1) + rút y từ pt (2) thay vào pt (1), ta được pt bậc hai 1 ẩn x, dễ rồi, tìm x rồi suy ra y
(2) + (3)
+ pt nào có nhân tử chung thì đặt nhân tử chung (thật ra chỉ có pt (2) của câu 2 là có nhân từ chung)
+ trong hệ, thấy biểu thức nào giống nhau thì đặt cho nó 1 ẩn phụ
VD hệ phương trình 3: đặt a= x+y ; b= căn (x+1)
+ khi đó ta nhận được một hệ phương trình bậc nhất hai ẩn, giải hpt đó rồi suy ra x và y
