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mấy bài này thì bạn cứ đặt ẩn phụ cho dễ nhìn hơn mà giải nhé
a, \(\hept{\begin{cases}\frac{1}{2x-y}+x+3y=\frac{3}{2}\\\frac{4}{2x-y}-5\left(x+3y\right)=-3\end{cases}}\)ĐK : \(2x\ne y\)
Đặt \(\frac{1}{2x-y}=t;x+3y=u\)hệ phương trình tương đương
\(\hept{\begin{cases}t+u=\frac{3}{2}\\4t-5u=-3\end{cases}\Leftrightarrow\hept{\begin{cases}4t+4u=6\\4t-5u=-3\end{cases}}\Leftrightarrow\hept{\begin{cases}9u=9\\4t=-3+5u\end{cases}}\Leftrightarrow\hept{\begin{cases}u=1\\t=\frac{-3+5}{4}=\frac{1}{2}\end{cases}}}\)
Theo cách đặt \(\hept{\begin{cases}x+3y=1\\\frac{1}{2x-y}=\frac{1}{2}\end{cases}\Leftrightarrow\hept{\begin{cases}x+3y=1\\2x-y=2\end{cases}}\Leftrightarrow\hept{\begin{cases}2x+6y=2\\2x-y=2\end{cases}\Leftrightarrow}\hept{\begin{cases}7y=4\\x=\frac{y+2}{2}\end{cases}\Leftrightarrow}\hept{\begin{cases}y=\frac{4}{7}\\x=\frac{9}{7}\end{cases}}}\)
Vậy hệ pt có một nghiệm (x;y) = (9/7;4/7)
a) \(2\sqrt{3x}-4\sqrt{3x}+27-2\sqrt{3x}=27-4\sqrt{3x}\)
b) \(3\sqrt{2x}-5\sqrt{8x}+7\sqrt{8x}+28=3\sqrt{2x}+2\sqrt{8x}+28=3\sqrt{2x}+4\sqrt{2x}+28=7\sqrt{2x}+28\)
c) \(\frac{2}{x^2-y^2}\sqrt{\frac{3\left(x+y\right)^2}{2}}=\frac{2}{\left(x-y\right)\left(x+y\right)}.\frac{\sqrt{3}\left|x+y\right|}{\sqrt{2}}=\frac{\sqrt{6}}{x-y}\)
d) \(\frac{2}{2a-1}\sqrt{5a^2\left(1-4x+4a^2\right)}=\frac{2}{2a-1}\sqrt{5a^2\left(2a-1\right)^2}=\frac{2}{2a-1}.\sqrt{5}\left|a\left(2a-1\right)\right|=2a\sqrt{5}\)
Thiếu ĐKXĐ : ..............
a) Ta có: \(2\sqrt{3x}-4\sqrt{3x}+27-2\sqrt{3x}\)
\(=27-4\sqrt{3x}\)
b) Ta có: \(3\sqrt{2x}-5\sqrt{8x}+7\sqrt{8x}+28\)
\(=3\sqrt{2x}-5.2\sqrt{2x}+7.2\sqrt{2x}+28\)
\(=3\sqrt{2x}-10\sqrt{2x}+14\sqrt{2x}+28\)
\(=7\sqrt{2x}+28\)
c) Ta có: \(\frac{2}{x^2-y^2}.\sqrt{\frac{3\left(x+y\right)^2}{2}}\)
\(=\sqrt{\frac{4}{\left(x-y\right)^2.\left(x+y\right)^2}.\frac{3\left(x+y\right)^2}{2}}\)
\(=\sqrt{\frac{2.3}{\left(x-y\right)^2}}\)
\(=\frac{1}{x-y}.\sqrt{6}\)
d) Ta có: \(\frac{2}{2a-1}.\sqrt{5a^2.\left(1-4a+4a^2\right)}\)
\(=\sqrt{\frac{4}{\left(2a-1\right)^2}.5a^2.\left(2a-1\right)^2}\)
\(=2a.\sqrt{5}\)
Câu 1: ĐKXĐ: ...
\(\Leftrightarrow4x\left(3x-1\right)+x-1=4x\sqrt{3x+1}\)
\(\Leftrightarrow12x^2-3x-1-4x\sqrt{3x+1}=0\)
\(\Leftrightarrow16x^2-\left(4x^2+4x\sqrt{3x+1}+3x+1\right)=0\)
\(\Leftrightarrow16x^2-\left(2x+\sqrt{3x+1}\right)^2=0\)
\(\Leftrightarrow\left(2x-\sqrt{3x+1}\right)\left(6x+\sqrt{3x+1}\right)=0\)
\(\Leftrightarrow...\)
Câu 2:
\(\Leftrightarrow\left\{{}\begin{matrix}x\left(x^2-4\right)=y^3+2y\\x^2-4=-3y^2\end{matrix}\right.\)
\(\Leftrightarrow x\left(-3y^2\right)=y^3+2y\)
\(\Leftrightarrow y\left(y^2+3xy+2\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}y=0\Rightarrow...\\y^2+3xy+2=0\left(1\right)\end{matrix}\right.\)
\(\left(1\right)\Leftrightarrow3xy=-y^2-2\Rightarrow x=\frac{-y^2-2}{3y}\)
\(\Rightarrow\left(\frac{y^2+2}{3y}\right)^2-1=3\left(1-y^2\right)\)
\(\Leftrightarrow\left(\frac{y^2-3y+2}{3y}\right)\left(\frac{y^2+3y+2}{3y}\right)=3\left(1-y^2\right)\)
\(\Leftrightarrow\frac{\left(y-1\right)\left(y-2\right)\left(y+1\right)\left(y+2\right)}{9y^2}=3\left(1-y^2\right)\)
\(\Leftrightarrow\frac{\left(y^2-1\right)\left(y^2-4\right)}{9y^2}=3\left(1-y^2\right)\)
\(\Leftrightarrow\left[{}\begin{matrix}y^2-1=0\\\frac{y^2-4}{9y^2}=-3\end{matrix}\right.\) \(\Leftrightarrow\left[{}\begin{matrix}y^2-1=0\\28y^2=4\end{matrix}\right.\)
\(3x-1+\frac{x-1}{4x}=\sqrt{3x+1}\)
\(\Leftrightarrow\frac{4x\left(3x-1\right)+x-1}{4x}=\sqrt{3x+1}\)
\(\Leftrightarrow\frac{12x^2-4x+x-1}{4x}=\sqrt{3x+1}\)
\(\Leftrightarrow\frac{12x^2-3x-1}{4x}=\sqrt{3x+1}\)
\(\Leftrightarrow\frac{\left(12x^2-3x-1\right)^2}{16x^2}=3x+1\)
\(\Leftrightarrow\left(12x^2-3x-1\right)^2=16x^2\left(3x+1\right)\)
\(\Leftrightarrow144x^4-120x^3-31x^2+6x+1=0\)
\(\Leftrightarrow144x^4-144x^3+24x^3-24x^2-7x^2+7x-x+1=0\)
\(\Leftrightarrow144x^3\left(x-1\right)+24x^2\left(x-1\right)+7x\left(x-1\right)-\left(x-1\right)=0\)
\(\Leftrightarrow\left(x-1\right)\left(144x^3+24x^2+7x-1\right)=0\)
Tìm được mỗi nghiệm thôi à :v
a.
\(\Leftrightarrow\left\{{}\begin{matrix}4xy+8x-6y-12=4xy-12x+54\\3xy-3x+3y-3=3xy+3y-12\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}20x-6y=66\\-3x=-9\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}x=3\\y=-1\end{matrix}\right.\)
b.
\(\Leftrightarrow\left\{{}\begin{matrix}y=1-x\\x^2+xy+3=0\end{matrix}\right.\)
\(\Leftrightarrow x^2+x\left(1-x\right)+3=0\)
\(\Leftrightarrow x+3=0\Rightarrow x=-3\Rightarrow y=4\)
c.
\(\Leftrightarrow\left\{{}\begin{matrix}y=\frac{2x-5}{3}\\x^2-y^2=40\end{matrix}\right.\)
\(\Rightarrow x^2-\left(\frac{2x-5}{3}\right)^2-40=0\)
\(\Leftrightarrow9x^2-\left(4x^2-20x+25\right)-360=0\)
\(\Leftrightarrow5x^2+20x-385=0\)
\(\Rightarrow\left[{}\begin{matrix}x=7\Rightarrow y=3\\x=-11\Rightarrow y=-9\end{matrix}\right.\)
d.
\(\Leftrightarrow\left\{{}\begin{matrix}y=\frac{36-3x}{2}\\\left(x-2\right)\left(y-3\right)=18\end{matrix}\right.\)
\(\Rightarrow\left(x-2\right)\left(\frac{36-3x}{2}-3\right)=18\)
\(\Leftrightarrow\left(x-2\right)\left(10-x\right)=12\)
\(\Leftrightarrow-x^2+12x-32=0\Rightarrow\left[{}\begin{matrix}x=4\Rightarrow y=12\\x=8\Rightarrow y=6\end{matrix}\right.\)
ĐKXĐ : \(x\ge1\)
PT đã cho tương đương với :
\(\sqrt{3x-2}+\sqrt{x-1}=\left[3x-2+2\sqrt{3x^2-5x+2}+x-1\right]-6\)
\(\Leftrightarrow\sqrt{3x-2}+\sqrt{x-1}=\left(\sqrt{3x-2}+\sqrt{x-1}\right)^2-6\)
Đặt \(\sqrt{3x-2}+\sqrt{x-1}=t\left(t\ge1\right)\)
Khi đó : \(t^2-t-6=0\Leftrightarrow\orbr{\begin{cases}t=3\\t=-2\left(loai\right)\end{cases}}\)
\(\Rightarrow\sqrt{3x-2}+\sqrt{x-1}=3\)
từ đó dễ dàng tìm được x
Làm tiếp bài của @Thanh Tùng DZ
Thay t=3 vào cách đặt ta được \(\sqrt{3x-2}+\sqrt{x-1}=3\left(3a\right)\)
Ta có \(\left(3a\right)\Leftrightarrow4x-3+2\sqrt{3x^2-5x+2}=9\)
\(\Leftrightarrow\sqrt{3x^2-5x+2}=6-2x\)
\(\Leftrightarrow\hept{\begin{cases}6-2x\ge0\\3x^2-5x+2=36-24x+4x^2\end{cases}}\)
\(\Leftrightarrow\hept{\begin{cases}x\le3\\x=2;x=17\end{cases}\Leftrightarrow x=2}\)
Bài 1:
ĐKXĐ: $-2\leq x\leq 2$
Đặt $\sqrt{2-x}=a; \sqrt{2+x}=b(a,b\geq 0)$
Ta có: \(\left\{\begin{matrix} a+b+ab=2\\ a^2+b^2=4\end{matrix}\right.\Leftrightarrow \left\{\begin{matrix} a+b=2-ab\\ (a+b)^2-2ab=4\end{matrix}\right.\)
\(\Rightarrow (2-ab)^2-2ab=4\)
\(\Leftrightarrow (ab)^2-6ab=0\Rightarrow \left[\begin{matrix} ab=0\\ ab=6\end{matrix}\right.\)
Nếu $ab=0\Rightarrow a+b=2$. Theo định lý Vi-et đảo thì $a,b$ là nghiệm của pt $X^2-2X=0\Rightarrow (a,b)=(0,2); (2,0)$
$\Rightarrow x=2$
Nếu $ab=6\Rightarrow a+b=-4$. Theo định lý Vi-et đảo thì $a,b$ là nghiệm của pt $X^2+4X+6=0$ (pt này vô nghiệm)
Vậy $x=2$
Bài 2:
ĐK: $x\geq \frac{-1}{3}
PT \(\Leftrightarrow \sqrt{5x+7}=\sqrt{x+3}+\sqrt{3x+1}\)
\(\Rightarrow 5x+7=4x+4+2\sqrt{(x+3)(3x+1)}\)
\(\Leftrightarrow x+3=2\sqrt{(x+3)(3x+1)}\)
\(\Leftrightarrow \sqrt{x+3}(\sqrt{x+3}-2\sqrt{3x+1})=0\)
Vì $x\geq \frac{-1}{3}$ nên $\sqrt{x+3}\neq 0$
Do đó $\sqrt{x+3}-2\sqrt{3x+1}=0$
$\Rightarrow x+3=4(3x+1)$
$\Rightarrow x=-\frac{1}{11}$ (thỏa mãn)
Vậy..........
Đặt \(\hept{\begin{cases}\left|3x-1\right|=a\\x-y=b\end{cases}}\Rightarrow\hept{\begin{cases}2a+b=\frac{7}{3}\\a+3b=2\end{cases}}\Leftrightarrow\hept{\begin{cases}a=1\\b=\frac{1}{3}\end{cases}}\)
hay ta có : \(\hept{\begin{cases}\left|3x-1\right|=1\\x-y=\frac{1}{3}\end{cases}}\Rightarrow\orbr{\begin{cases}x=0\Rightarrow y=-\frac{1}{3}\\x=\frac{2}{3}\Rightarrow y=\frac{1}{3}\end{cases}}\)
\(\hept{\begin{cases}2\left|3x-1\right|+x-y=\frac{7}{3}\\\left|3x-1\right|+3\left(x-y\right)=2\end{cases}}\)(1)
Với x < 1/3 \(\left(1\right)\Leftrightarrow\hept{\begin{cases}2-6x+x-y=\frac{7}{3}\\1-3x+3x-3y=2\end{cases}}\Leftrightarrow\hept{\begin{cases}-5x-y=\frac{1}{3}\\-3y=1\end{cases}}\Leftrightarrow\hept{\begin{cases}x=0\left(tm\right)\\y=-\frac{1}{3}\end{cases}}\)
Với x >= 1/3 \(\left(1\right)\Leftrightarrow\hept{\begin{cases}6x-2+x-y=\frac{7}{3}\\3x-1+3x-3y=2\end{cases}}\Leftrightarrow\hept{\begin{cases}7x-y=\frac{13}{3}\\2x-y=1\end{cases}}\Leftrightarrow\hept{\begin{cases}5x=\frac{10}{3}\\2x-y=1\end{cases}}\Leftrightarrow\hept{\begin{cases}x=\frac{2}{3}\left(tm\right)\\y=\frac{1}{3}\end{cases}}\)