`a)2x(x-5)-x(3+2x)=26`
`⇔2x.x+2x.(-5)+(-x).3+(-x).2x=26`
`⇔2x^2-10x-3x-2x^2=26`
`⇔-13x=26`
`⇔x=26:(-13)
`⇔x=-2`
Vậy `S=-2`
`b)49x^2-81=0`
`⇔(7x)^2-9^2=0`
`⇔(7x-9)(7x+9)=0`
`⇔`\(\left[ \begin{array}{l}7x-9=0\\7x+9=0\end{array} \right.\)
`⇔`\(\left[ \begin{array}{l}7x=9\\7x=-9\end{array} \right.\)
`⇔`\(\left[ \begin{array}{l}x=\dfrac{9}{7}\\x=\dfrac{-9}{7}\end{array} \right.\)
Vậy `S={9/7;-9/7}.`
