x2-5x-4x+20=0
x2-4x-5x+20=0
x(x-4)-5(x-4)=0
(x-4)(x-5)=0
\(\Rightarrow\hept{\begin{cases}x-4=0\Rightarrow x=4\\x-5=0\Rightarrow x=5\end{cases}}\)
tớ hỏi nốt nha
tính nhanh:202-192+182-172+...+22-12
\(x^2-9x+20=0\)
\(\Rightarrow x^2-2.x.\frac{9}{2}+\frac{81}{4}-\frac{1}{4}=0\)
\(\Rightarrow x^2-2.x.\frac{9}{2}+\left(\frac{9}{2}\right)^2-\frac{1}{4}=0\)
\(\Rightarrow\left(x-\frac{9}{2}\right)^2-\left(\frac{1}{2}\right)^2=0\)
\(\Rightarrow\left(x-\frac{9}{2}-\frac{1}{2}\right)\left(x-\frac{9}{2}+\frac{1}{2}\right)=0\)
\(\Rightarrow\left(x-5\right)\left(x-4\right)=0\)
\(\Rightarrow\orbr{\begin{cases}x-5=0\\x-4=0\end{cases}}\Rightarrow\orbr{\begin{cases}x=5\\x=4\end{cases}}\)
Vậy \(x=5\)hoặc \(x=4\)
