\(x^2-x-20=0\)
\(\Leftrightarrow x^2-5x+4x-20=0\)
\(\Leftrightarrow x\left(x-5\right)+4\left(x-5\right)=0\)
\(\left(x-5\right)\left(x+4\right)=0\)
\(\Leftrightarrow\orbr{\begin{cases}x-5=0\\x+4=0\end{cases}\Leftrightarrow\orbr{\begin{cases}x=5\\x=-4\end{cases}}}\)
cn lại lm tg tự nha bn
=.= hok tốt!!
`Answer:`
\(x^2-x-20=0\)
\(\Leftrightarrow x^2-2x.\frac{1}{2}+\frac{1}{4}-\frac{81}{4}=0\)
\(\Leftrightarrow\left(x-\frac{1}{2}\right)^2=\frac{81}{4}\)
\(\Leftrightarrow\left(x-\frac{1}{2}\right)^2=\left(\frac{9}{2}\right)^2\)
\(\Leftrightarrow\orbr{\begin{cases}x-\frac{1}{2}=\frac{9}{2}\\x-\frac{1}{2}=-\frac{9}{2}\end{cases}}\)
\(\Leftrightarrow\orbr{\begin{cases}x=5\\x=-4\end{cases}}\)
\(x^2+80x-20=0\)
\(\Leftrightarrow x^2+2.40x+1600-1620=0\)
\(\Leftrightarrow\left(x+40\right)^2-\sqrt{1620}=0\)
\(\Leftrightarrow\left(x+40\right)^2=18\sqrt{5}\)
\(\Leftrightarrow\orbr{\begin{cases}x+40=18\sqrt{5}\\x+40=-18\sqrt{5}\end{cases}}\)
\(\Leftrightarrow\orbr{\begin{cases}x=18\sqrt{5}-40\\x=-18\sqrt{5}-40\end{cases}}\)
\(x^2+5x-6=0\)
\(\Leftrightarrow x^2-x+6x-6=0\)
\(\Leftrightarrow x.\left(x-1\right)+6.\left(x-1\right)=0\)
\(\Leftrightarrow\left(x+6\right)\left(x-1\right)=0\)
\(\Leftrightarrow\orbr{\begin{cases}x+6=0\\x-1=0\end{cases}}\)
\(\Leftrightarrow\orbr{\begin{cases}x=-6\\x=1\end{cases}}\)
