;-;!!!

để pt trên có 2 nghiệm pb thì \(\Delta'>0\)

<=> \(m^2+6m+9-4m-12>0\)

<=>\(m^2+2m-3>0\)

<=>\(\left(m-1\right)\left(m+3\right)>0\)

<=>\(\left[{}\begin{matrix}m>1\\m< -3\end{matrix}\right.\)

cho \(x_1,x_2\)  là 2 nghiệm của pt và \(x_1< x_2\)

cần chứng minh \(x_1>-1\)

<=>\(-m-3-\sqrt{m^2+2m-3}>-1\)

<=>\(\sqrt{m^2+2m-3}>m+2\)

<=>\(\left[{}\begin{matrix}m^2+2m-3>m^2+4m+4\\m^2+2m-3>-m^2-4m-4\end{matrix}\right.\)

<=>\(\left[{}\begin{matrix}2m+7< 0\\2m^2+6m+1>0\end{matrix}\right.\)

<=>\(\left[{}\begin{matrix}m< \dfrac{-7}{2}\\m>\dfrac{-3+\sqrt{7}}{2}\\m< \dfrac{-3-\sqrt{7}}{2}\end{matrix}\right.\)

so với điều kiện ở đè bài =>\(m< \dfrac{-7}{2}\)thỏa yêu câu đề bài 

KL: để pt có 2 nghiệm pb đều lớn hơn -1 thì \(m< \dfrac{-7}{2}\)

\(\Delta'=\left(m+3\right)^2-\left(4m+12\right)=m^2+2m-3>0\Rightarrow\left[{}\begin{matrix}m>1\\m< -3\end{matrix}\right.\)

Theo hệ thức Viet: \(\left\{{}\begin{matrix}x_1+x_2=-2\left(m+3\right)\\x_1x_2=4m+12\end{matrix}\right.\)

Pt có 2 nghiệm lớn hơn -1 khi: \(-1< x_1< x_2\Leftrightarrow\left\{{}\begin{matrix}\left(x_1+1\right)\left(x_2+1\right)>0\\\dfrac{x_1+x_2}{2}>-1\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}x_1x_2+x_1+x_2+1>0\\x_1+x_2>-2\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}4m+12-2\left(m+3\right)+1>0\\-2\left(m+3\right)>-2\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}m>-\dfrac{7}{2}\\m< -2\end{matrix}\right.\) \(\Rightarrow-\dfrac{7}{2}< m< -2\)

Kết hợp điều kiện ban đầu \(\Rightarrow-\dfrac{7}{2}< m< -3\)

1, ĐKXĐ:\(x\ne2,y\ne1\)

Đặt `1/(x-2)` = a, `1/(y-1)` = b

\(Hệ.\Leftrightarrow\left\{{}\begin{matrix}a+b=2\\2a-3b=1\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}a=\dfrac{7}{5}\\b=\dfrac{3}{5}\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}\dfrac{1}{x-2}=\dfrac{7}{5}\\\dfrac{1}{y-1}=\dfrac{3}{5}\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}7x-14=5\\3y-3=5\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{19}{7}\\y=\dfrac{8}{3}\end{matrix}\right.\)\(2,\Delta'=\left[-\left(m+1\right)\right]^2-4m=m^2+2m+1-4m=m^2-2m+1=\left(m-1\right)^2\ge0\)

Để pt có 2 nghiệm phân biệt thì \(\Delta'>0\Leftrightarrow\left(m-1\right)^2>0\Leftrightarrow m-1\ne0\Leftrightarrow m\ne1\)

b, Theo Vi-ét:\(\left\{{}\begin{matrix}x_1+x_2=2m+2\\x_1x_2=4m\end{matrix}\right.\)

\(\left(x_1-x_2\right)^2-x_1x_2=3\\ \Leftrightarrow\left(x_1+x_2\right)^2-5x_1x_2=3\\ \Leftrightarrow\left(2m+2\right)^2-5.4m-3=0\\ \Leftrightarrow4m^2+8m+4-20m-3=0\\ \Leftrightarrow4m^2-12m+1=0\\ \Leftrightarrow\left[{}\begin{matrix}x=\dfrac{3+2\sqrt{2}}{2}\\x=\dfrac{3-2\sqrt{2}}{2}\end{matrix}\right.\)

d: Ta có: \(\text{Δ}=\left(m+1\right)^2-4\cdot2\cdot\left(m+3\right)\)

\(=m^2+2m+1-8m-24\)

\(=m^2-6m-23\)

\(=m^2-6m+9-32\)

\(=\left(m-3\right)^2-32\)

Để phương trình có hai nghiệm phân biệt thì \(\left(m-3\right)^2>32\)

\(\Leftrightarrow\left[{}\begin{matrix}m-3>4\sqrt{2}\\m-3< -4\sqrt{2}\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}m>4\sqrt{2}+3\\m< -4\sqrt{2}+3\end{matrix}\right.\)

Áp dụng hệ thức Vi-et, ta được:

\(\left\{{}\begin{matrix}x_1+x_2=\dfrac{m+1}{2}\\x_1x_2=\dfrac{m+3}{2}\end{matrix}\right.\)

Ta có: \(\left\{{}\begin{matrix}x_1+x_2=\dfrac{m+1}{2}\\x_1-x_2=1\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}2x_1=\dfrac{m+3}{2}\\x_2=x_1-1\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}x_1=\dfrac{m+3}{4}\\x_2=\dfrac{m+3}{4}-\dfrac{4}{4}=\dfrac{m-1}{4}\end{matrix}\right.\)

Ta có: \(x_1x_2=\dfrac{m+3}{2}\)

\(\Leftrightarrow\dfrac{\left(m+3\right)\left(m-1\right)}{16}=\dfrac{m+3}{2}\)

\(\Leftrightarrow\left(m+3\right)\left(m-1\right)=8\left(m+3\right)\)

\(\Leftrightarrow\left(m+3\right)\left(m-9\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}m=-3\\m=9\end{matrix}\right.\)