\(K=\left(\frac{1-x}{x+3}-\frac{x+3}{x-1}\right):\left(\frac{x+3}{x-1}+\frac{1-x}{x+3}\right)\)
\(\Leftrightarrow K=\frac{-\left(x-1\right)+\left(x+3\right)^2}{\left(x+3\right)\left(x-1\right)}:\frac{\left(x+3\right)^2-\left(x-1\right)^2}{\left(x-1\right)\left(x+3\right)}\)
\(\Leftrightarrow K=\frac{-x^2+2x-1-x^2-6x-9}{\left(x+3\right)\left(x-1\right)}\cdot\frac{\left(x-1\right)\left(x+3\right)}{x^2+6x+9-x^2+2x-1}\)
\(\Leftrightarrow K=\frac{-2x^2-4x-10}{8x+8}\)
Phần còn lại tự tìm x thỏa mãn