Viet: \(x_1+x_2=1\)

Mà \(x_1-x_2=7\Leftrightarrow\left\{{}\begin{matrix}x_1=-3\\x_2=-4\end{matrix}\right.\)

Vậy ...

a: Ta có: \(\left\{{}\begin{matrix}3x+2y=14\\5x+3y=1\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}15x+10y=70\\15x+9y=3\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}y=67\\3x=14-2y=14-2\cdot67=-120\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=-40\\y=67\end{matrix}\right.\)

b: Ta có: \(\left\{{}\begin{matrix}-x+2y-6=0\\5x-3y-5=0\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}-x+2y=6\\5x-3y=5\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}-5x+10y=30\\5x-3y=5\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}7y=35\\2y-x=6\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=5\\y=4\end{matrix}\right.\)

x2 – 5x + 6 = 0

⇔ x2 – 2x – 3x + 6 = 0

(Tách để xuất hiện nhân tử chung)

⇔ (x2 – 2x) – (3x – 6) = 0

⇔ x(x – 2) – 3(x – 2) = 0

⇔(x – 3)(x – 2) = 0

⇔ x – 3 = 0 hoặc x – 2 = 0

+ x – 3 = 0 ⇔ x = 3.

+ x – 2 = 0 ⇔ x = 2.

Vậy tập nghiệm của phương trình là S = {2; 3}.

Chọn đáp án A

c: =>(x+2)(x+3)(x-5)(x-6)=180

=>(x^2-3x-10)(x^2-3x-18)=180

=>(x^2-3x)^2-28(x^2-3x)=0

=>x(x-3)(x-7)(x+4)=0

=>\(x\in\left\{0;3;7;-4\right\}\)

c: =>(x-3)(x+2)(2x+1)(3x-1)=0

=>\(x\in\left\{3;-2;-\dfrac{1}{2};\dfrac{1}{3}\right\}\)

\(6x^4+5x^3-38x^2+5x+6=0\\ \Leftrightarrow6x^4+20x^3+6x^2-15x^3-50x^2-15x+6x^2+20x+6=0\\ \Leftrightarrow2x^2\left(3x^2+10x+3\right)-5x\left(3x^2+10x+3\right)+2\left(3x^2+10x+3\right)=0\\ \Leftrightarrow\left(3x^2+10x+3\right)\left(2x^2-5x+2\right)=0\\ \Leftrightarrow\left(3x^2+x+9x+3\right)\left(2x^2-x-4x+2\right)=0\\ \Leftrightarrow\left[x\left(3x+1\right)+3\left(3x+1\right)\right]\left[x\left(2x-1\right)-2\left(2x-1\right)\right]=0\\ \Leftrightarrow\left(3x+1\right)\left(x+3\right)\left(2x-1\right)\left(x-2\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}3x+1=0\\x+3=0\\2x-1=0\\x-2=0\end{matrix}\right.\\ \Leftrightarrow\left[{}\begin{matrix}x=\dfrac{-1}{3}\\x=-3\\x=\dfrac{1}{2}\\x=2\end{matrix}\right.\)