Ta có:\(\dfrac{x-1}{79}=\dfrac{-28}{1-x}\)
\(\Rightarrow\left(x-1\right)\left(1-x\right)=-28.79\)
\(\Rightarrow\left(x-1\right).1-\left(x-1\right)x=-2212\)
\(\Rightarrow x-1-x^2+x=-2212\)
\(\Rightarrow x+x-x^2=-2211\)
\(\Rightarrow2x-x^2=-2211\)
\(\Rightarrow x\left(2-x\right)=-2211\)
...
\(\dfrac{x-1}{79}=\dfrac{-28}{1-x}\)
\(\Leftrightarrow\left(x-1\right)\left(1-x\right)=\left(-28\right).79\)
\(\Leftrightarrow x\left(1-x\right)-1\left(1-x\right)=-2212\)
\(\Leftrightarrow x-x^2-1-x=-2212\)
\(\Leftrightarrow-x^2-1=-2212\)
\(\Leftrightarrow-x^2=-2211\)
\(\Leftrightarrow x^2=2211\)
\(\Leftrightarrow x=\sqrt{2211}\)
