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\(\left(12x+7\right)^2\left(3x+2\right)\left(2x+1\right)=3\)
\(\Leftrightarrow\left(12x+7\right)^2\cdot4\left(3x+2\right)\cdot6\left(2x+1\right)=3\cdot4\cdot6\)
\(\Leftrightarrow\left(12x+7\right)^2\left(12x+8\right)\left(12x+6\right)=72\) (1)
Đặt 12x + 7 = a
(1) \(\Leftrightarrow a^2\left(a+1\right)\left(a-1\right)=72\)
\(\Leftrightarrow a^2\left(a^2-1\right)=72\) (2)
Đặt \(a^2=b\)
(2) \(\Leftrightarrow b\left(b-1\right)=72\)
\(\Leftrightarrow b^2-b-72=0\)
\(\Leftrightarrow b^2+8b-9b-72=0\)
\(\Leftrightarrow b\left(b+8\right)-9\left(b+8\right)=0\)
\(\Leftrightarrow\left(b-9\right)\left(b+8\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}b-9=0\\b+8=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}b=9\\b=-8\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}a^2=9\Leftrightarrow a=\pm3\\a^2=-8\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}12x+7=3\\12x+7=-3\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}12x=-4\\12x=-10\end{matrix}\right.\Leftrightarrow}\left[{}\begin{matrix}x=-\dfrac{1}{3}\\x=-\dfrac{5}{6}\end{matrix}\right.\)
\(\left(12x+7\right)^2\left(3x+2\right)\left(2x+1\right)=3\)
\(\Leftrightarrow\left(144x^2+168x+49\right)\left(6x^2+7x+2\right)=3\)
\(\Leftrightarrow\left(144x^2+168x+49\right)\left(144x^2+168+48\right)=72\)
Đặt \(144x^2+168x+48=u\)
\(\Rightarrow144x^2+168x+49=u+1\left(1\right)\)
Do đó: \(u\left(u+1\right)=72\Leftrightarrow u^2+u-72=0\)
\(\Leftrightarrow\left(u-8\right)\left(u+9\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}u-8=0\\u+9=0\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}u=8\\u=-9\end{matrix}\right.\)
Với \(u=8;u=-9\) bạn thay vào (1) và tìm x nha.
a)\(\frac{2-x}{2007}-1=\frac{1-x}{2008}-\frac{x}{2009}\)
\(\Leftrightarrow\frac{2-x}{2007}-1+2=\frac{1-x}{2008}+1-\frac{x}{2009}+1\)
\(\Leftrightarrow\frac{2-x}{2007}+\frac{2007}{2007}=\frac{1-x}{2008}+\frac{2008}{2008}-\frac{x}{2009}+\frac{2009}{2009}\)
\(\Leftrightarrow\frac{2009-x}{2007}=\frac{2009-x}{2008}-\frac{2009-x}{2009}\)
\(\Leftrightarrow\frac{2009-x}{2007}-\frac{2009-x}{2008}+\frac{2009-x}{2009}=0\)
\(\Leftrightarrow\left(2009-x\right)\left(\frac{1}{2007}-\frac{1}{2008}+\frac{1}{2009}\right)=0\)
\(\Leftrightarrow2009-x=0\).Do \(\frac{1}{2007}-\frac{1}{2008}+\frac{1}{2009}\ne0\)
\(\Leftrightarrow x=2009\)
b)\(\left(12x+7\right)^2\left(3x+2\right)\left(2x+1\right)=3\)
\(\Leftrightarrow\left(12^2x^2+2\cdot12\cdot7x+7^2\right)\left(6x^2+7x+2\right)-3=0\)
\(\Leftrightarrow\left[24\left(6x^2+7x+2\right)+1\right]\left(6x^2+7x+2\right)-3=0\)
Đặt \(t=6x^2+7x+2\) ta có:
\(\left(24t+1\right)t-3=0\)\(\Leftrightarrow12t^2+t-3=0\)
Suy ra t rồi tìm đc x
a)\(3\left(x^4+x^2+1\right)=\left(x^2+x+1\right)^2\)
Cauchy-schwarz:
\(\left(1+1+1\right)\left(x^4+x^2+1\right)\ge\left(x^2+x+1\right)^2\)
"="<=>\(x=1\)
b)\(x\left(x+1\right)\left(x-1\right)\left(x+2\right)=24\)
\(\Leftrightarrow\left(x^2+x\right)\left(x^2+x-2\right)=24\)
\(x^2+x-1=t\)
\(\Rightarrow\left(t-1\right)\left(t+1\right)=24\)
\(\Leftrightarrow t^2-25=0\)
\(\Leftrightarrow t=\pm5\)
t=5\(\Leftrightarrow x^2+x-1=5\)
\(\Leftrightarrow\left(x-2\right)\left(x+3\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x=2\\x=-3\end{matrix}\right.\)
t=-5<=> pt vô nghiệm
Làm cho bạn 1 con thôi dài quá trôi hết màn hình:
c) có vẻ khó nhất (con khác tương tự)
đặt 2x+2=t=> x+1=t/2
\(\left(t-1\right).\left(\frac{t}{2}\right)^{^2}.\left(t+1\right)=18\Leftrightarrow\left(t^2-1\right)t^2=4.18\)
\(t^4-t^2=4.18\Leftrightarrow y^2-2.\frac{1}{2}y+\frac{1}{4}=4.18+\frac{1}{4}=\frac{16.18+1}{4}=\left(\frac{17}{2}\right)^2\)
<=> \(\left(y-\frac{1}{2}\right)^{^2}=\left(\frac{17}{2}\right)^2\Rightarrow\left[\begin{matrix}y=\frac{1}{2}-\frac{17}{2}=-8\\y=\frac{1}{2}+\frac{17}{2}=9\end{matrix}\right.\Rightarrow\left[\begin{matrix}2x+2=-8\Rightarrow x=-5\\2x+2=9\Rightarrow x=\frac{7}{2}\end{matrix}\right.\)
(12x+7)2(3x+2)(2x+1)=3
<=> (144x2+168x+49)(6x2+7x+2)=3
<=>(144x2+168x+49)(144x+168+48)=72
Đặt 144x2+168x+48=t
=> 144x2+168x+49=t+1(*)
Do đó phương trình đã cho là
(t+1)t=72
<=> t2+t-72=0
<=> (t-8)(t+9)=0
<=>\(\left[{}\begin{matrix}t-8=0\\t+9=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}t=8\\t=-9\end{matrix}\right.\)
Bạn tự thay t vào (*) rồi tìm x nha