\(8x^2-\left(3x-1\right)^2-\left(x+2\right)\left(5-x\right)=20\) (1)
\(\Leftrightarrow8x^2-\left(3x-1\right)\left(3x-1\right)-\left(x+2\right)\left(5-x\right)=20\)
\(\Leftrightarrow8x^2-\left[3x\left(3x-1\right)-1\left(3x-1\right)\right]-\left[x\left(5-x\right)+2\left(5-x\right)\right]=20\)
\(\Leftrightarrow8x^2-\left(9x^2-3x-3x+1\right)-\left(5x-x^2+10-2x\right)=20\)
\(\Leftrightarrow8x^2-9x^2+3x+3x-1-5x+x^2-10+2x=20\)
\(\Leftrightarrow3x-11=20\)
\(\Leftrightarrow3x=20+11\)
\(\Leftrightarrow3x=31\)
\(\Leftrightarrow x=\dfrac{31}{3}\)
Vậy tập nghiệm phương trình (1) \(x=\left\{\dfrac{31}{3}\right\}\)
\(8x^2-\left(3x-1\right)^2-\left(x+2\right)\left(5-x\right)=20\)
\(\Leftrightarrow8x^2-9x^2+6x-1-5x+x^2-10+2x=20\)
\(\Leftrightarrow3x-11=20\)
\(\Leftrightarrow3x=31\)
\(\Leftrightarrow x=\dfrac{31}{3}\)
Vậy ...
8x2-( 3x-1)2-(x+2).(5-x)=20
8x2-9x2-1- x.(5-x)+2.(5-x)=20
8x2-9x2-1-5x-x2-10+2x =20
(8x2-9x2-x2)+(-1-10)+(-5x+2x)=10
-11 -3x =10
-3x = -21
x= 7
Chúc bạn học tốt 
8x2-(3x-1)2-(x+2)(5-x)=20
8x2-3x2-12-x(5-x)-2(5-x)=20
8x2-9x2-1-5x+x2-10+2x=20
8x2-9x2+x2-11-5x+2x=20
3x-11=20
3x=31
x=31/3
