a: \(\Leftrightarrow-x^2-3\left(x-5\right)=x\left(4-x\right)\)
\(\Leftrightarrow-x^2-3x+15=4x-x^2\)
=>-3x+15=4x
=>-7x=-15
hay x=15/7(nhận)
b: \(\Leftrightarrow\left(x+1\right)\left(x+2\right)+\left(x-1\right)\left(x-2\right)=2x^2+4\)
\(\Leftrightarrow x^2+3x+2+x^2-3x+2=2x^2+4\)
=>4=4(luôn đúng)
Vậy: S={x|\(x\notin\left\{2;-2\right\}\)}