Giải phương trình:
\(\left(x+2\right)\left(x+4\right)\left(x+6\right)\left(x+12\right)=165x^2\)
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BV
Phân tích thành nhân tử:
\(\left(x+2\right)\left(x+4\right)\left(x+6\right)\left(x+12\right)-165x^2\)
1
LH
27 tháng 5 2018
\(\left(x+2\right)\left(x+4\right)\left(x+6\right)\left(x+12\right)-165x^2\)
\(=\left[\left(x+2\right)\left(x+12\right)\right]\left[\left(x+4\right)\left(x+6\right)\right]-165x^2\)
\(=\left(x^2+14x+24\right)\left(x^2+10x+24\right)-165x^2\)
\(=\left(x^2+12x+24+2x\right)\left(x^2+12x+24-2x\right)-165x^2\)
\(=\left(x^2+12x+24\right)^2-4x^2-165x^2\)
\(=\left(x^2+12x+24\right)^2-169x^2\)
\(=\left(x^2+12x+24-13x\right)\left(x^2+12x+24+13x\right)\)
\(=\left(x^2-x+24\right)\left(x^2+25x+24\right)\)
\(=\left(x^2-x+24\right)\left(x^2+x+24x+24\right)\)
\(=\left(x^2-x+24\right)\left[x\left(x+1\right)+24\left(x+1\right)\right]\)
\(=\left(x^2-x+24\right)\left(x+1\right)\left(x+24\right)\)
KT
24 tháng 1 2018
b) Đặt \(x-7=a\) ta có:
\(\left(a+1\right)^4+\left(a-1\right)^4=16\)
\(\Leftrightarrow\)\(a^4+4a^3+6a^2+4a+1+a^4-4a^3+6a^2-4a+1=16\)
\(\Leftrightarrow\)\(2a^4+12a^2+2-16=0\)
\(\Leftrightarrow\)\(2\left(a^4+6a^2-7\right)=0\)
\(\Leftrightarrow\)\(a^4+6a^2-7=0\)
\(\Leftrightarrow\)\(\left(a-1\right)\left(a+1\right)\left(a^2+7\right)=0\)
Vì \(a^2+7>0\) nên \(\orbr{\begin{cases}a-1=0\\a+1=0\end{cases}}\)
Thay trở lại ta có: \(\orbr{\begin{cases}x-8=0\\x-6=0\end{cases}}\) \(\Leftrightarrow\)\(\orbr{\begin{cases}x=8\\x=6\end{cases}}\)
Vậy...
TH
0
18 tháng 12 2021
PT tương đương
\(\left(x^2+7x+6\right)\left(x^2+5x+6\right)=\dfrac{-3x^2}{4}\)
Xét \(x=0\Rightarrow6.6=0\)(vô lý)
Xét \(x\ne0\). Ta chia 2 vế của PT cho \(x^2\ne0\). PT tương đương
\(\left(x+\dfrac{6}{x}+7\right)\left(x+\dfrac{6}{x}+5\right)=\dfrac{-3}{4}\)
Đặt \(x+\dfrac{6}{x}+5=t\)
PT\(\Leftrightarrow t\left(t+2\right)=\dfrac{-3}{4}\Leftrightarrow t^2+2t+1=\dfrac{1}{4}\)
\(\Leftrightarrow\left(t+1\right)^2=\dfrac{1}{4}\Leftrightarrow\left[{}\begin{matrix}t+1=\dfrac{-1}{2}\\t+1=\dfrac{1}{2}\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}t=\dfrac{-3}{2}\\t=\dfrac{-1}{2}\end{matrix}\right.\)
Đến đây bạn thay vào là tìm được nghiệm nhé.
28 tháng 8 2021
b: Ta có: \(\left(x+2\right)\left(x+3\right)\left(x+4\right)\left(x+5\right)-24=0\)
\(\Leftrightarrow\left(x^2+7x+10\right)\left(x^2+7x+12\right)-24=0\)
\(\Leftrightarrow\left(x^2+7x\right)^2+22\left(x^2+7x\right)+120-24=0\)
\(\Leftrightarrow x^2+7x+6=0\)
\(\Leftrightarrow\left(x+1\right)\left(x+6\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x=-1\\x=-6\end{matrix}\right.\)
20 tháng 3 2020
\(\frac{\left(x-2\right)^2}{12}-\frac{\left(x+1\right)^2}{21}=\frac{\left(x-4\right)\left(x-6\right)}{28}\)
<=> \(\frac{7\left(x^2-4x+4\right)}{84}-\frac{4\left(x^2+2x+1\right)}{84}=\frac{3\left(x^2-10x+24\right)}{84}\)
<=> 7x2 - 28x + 28 - 4x2 - 8x - 4 = 3x2 - 30x + 72
<=> 3x^2 - 36x - 3x^2 + 30x = 72 - 24
<=> -6x = 48
<=> x = -8
Vậy S = {-8}
22 tháng 3 2021
ĐKXĐ: \(x\notin\left\{-1;-2;-3;-4\right\}\)
Ta có: \(\dfrac{1}{\left(x+1\right)\left(x+2\right)}+\dfrac{1}{\left(x+2\right)\left(x+3\right)}+\dfrac{1}{\left(x+3\right)\left(x+4\right)}=\dfrac{1}{6}\)
\(\Leftrightarrow\dfrac{1}{x+1}-\dfrac{1}{x+2}+\dfrac{1}{x+2}-\dfrac{1}{x+3}+\dfrac{1}{x+3}-\dfrac{1}{x+4}=\dfrac{1}{6}\)
\(\Leftrightarrow\dfrac{1}{x+1}-\dfrac{1}{x+4}=\dfrac{1}{6}\)
\(\Leftrightarrow\dfrac{x+4}{\left(x+1\right)\left(x+4\right)}-\dfrac{x+1}{\left(x+1\right)\left(x+4\right)}=\dfrac{1}{6}\)
\(\Leftrightarrow\dfrac{x+4-x-1}{\left(x+1\right)\left(x+4\right)}=\dfrac{x^2+5x+4}{6\left(x+1\right)\left(x+4\right)}\)
\(\Leftrightarrow\dfrac{18}{6\left(x+1\right)\left(x+4\right)}=\dfrac{x^2+5x+4}{6\left(x+1\right)\left(x+4\right)}\)
Suy ra: \(x^2+5x+4=18\)
\(\Leftrightarrow x^2+5x-14=0\)
\(\Leftrightarrow x^2+7x-2x-14=0\)
\(\Leftrightarrow x\left(x+7\right)-2\left(x+7\right)=0\)
\(\Leftrightarrow\left(x+7\right)\left(x-2\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x+7=0\\x-2=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=-7\left(nhận\right)\\x=2\left(nhận\right)\end{matrix}\right.\)
Vậy: S={-7;2}
\(\left(x+2\right)\left(x+12\right)\left(x+4\right)\left(x+6\right)=165x^2\)
\(\Leftrightarrow\left(x^2+14x+24\right)\left(x^2+10x+24\right)=165x^2\)
Nhận thấy \(x=0\) không phải nghiệm, pt đã cho tương đương:
\(\left(x+14+\dfrac{24}{x}\right)\left(x+10+\dfrac{24}{x}\right)=165\)
Đặt \(x+\dfrac{24}{x}+10=t\):
\(t\left(t+4\right)-165=0\Leftrightarrow t^2+4t-165=0\Rightarrow\left[{}\begin{matrix}t=11\\t=-15\end{matrix}\right.\)
TH1: \(x+\dfrac{24}{x}+10=11\Leftrightarrow x^2-x+24=0\) (vô nghiệm)
TH2: \(x+\dfrac{24}{x}+10=-15\Leftrightarrow x^2+25x+24=0\)
\(\Rightarrow\left[{}\begin{matrix}x=-1\\x=-24\end{matrix}\right.\)