a: =>x^2+6x-3x-18=0
=>(x+6)(x-3)=0
=>x=3 hoặc x=-6
b: =>8x^2+28x+2x+7=0
=>4x(2x+7)+(2x+7)=0
=>(2x+7)(4x+1)=0
=>x=-7/2 hoặc x=-1/4
c: =>x(x^2+11x+30)=0
=>x(x+6)(x+5)=0
=>x=0;x=-6;x=-5
e: =>(x-3y)^2=9
=>x-3y=3 hoặc x-3y=-3
f: =>(x+3)(5-2x)=0
=>x=5/2 hoặc x=-3
\(a,x^2+3x-18=0\\ \Leftrightarrow x^2+6x-3x-18=0\\ \Leftrightarrow x\left(x+6\right)-3\left(x+6\right)=0\\ \Leftrightarrow\left(x-3\right)\left(x+6\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=3\\x=-6\end{matrix}\right.\)
\(b,8x^2+30x+7=0\\ \Leftrightarrow8x^2+2x+28x+7=0\\ \Leftrightarrow2x\left(4x+1\right)+7\left(4x+1\right)=0\\ \Leftrightarrow\left(2x+7\right)\left(4x+1\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=-\dfrac{7}{2}\\x=-\dfrac{1}{4}\end{matrix}\right.\)
\(c,x^3+11x^2+30x=0\\ \Leftrightarrow x\left(x^2+5x+6x+30\right)=0\\ \Leftrightarrow x\left(x+5\right)\left(x+6\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=0\\x=-5\\x=-6\end{matrix}\right.\)
\(d,\left(x^2-4x\right)-8\left(x^2-4x\right)+15=0\\ \Leftrightarrow7\left(x^2-4x\right)=15\\ \Leftrightarrow\left[{}\begin{matrix}x=\dfrac{14+\sqrt{301}}{7}\\x=\dfrac{14-\sqrt{301}}{7}\end{matrix}\right.\)
\(e,x^2-6xy+9y^2-9\\ =\left(x-3y\right)^2-3^2\\ =\left(x-3y+3\right)\left(x-3y-3\right)\)
\(5\left(x+3\right)-2x\left(3+x\right)=0\\ \Leftrightarrow\left(x+3\right)\left(5-2x\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=-3\\x=\dfrac{5}{2}\end{matrix}\right.\)
