13/ (x-2)²- x(x-5)- 3=0
<=> x²- 4x+ 4- x²+ 5x- 3=0
<=> x+1=0
<=> x=-1
Vậy x= -1
14/ (3x + 1) (3x - 1) - (x - 2)(x² + 2x + 4) = x (6 -x²)
<=> 9x²- 1 -(x³+ 2x²+ 4x- 2x²- 4x-8)= 6x- x³
<=> 9x²- 1- x³- 2x²- 4x+ 2x²+ 4x+ 8- 6x + x³ =0
<=> 9x²- 6x+ 7 =0
<=> 9x²- 6x+ 1+ 6=0
<=> (3x-1)²=-6
=> pt vô nghiệm (vì (3x-1)² ≥0 với mọi x)
15/ 5x(x - 3)² + 5(x - 4) (x + 4) - 5x³ = -5x(6x - 1)
<=> 5x(x²- 6x+9)+ 5(x²- 16)- 5x³= -30x²+ 5x
<=> 5x³- 30x²+ 45x+ 5x²- 80- 5x³+ 30x²- 5x=0
<=> 5x²+ 40x- 80=0
<=> x²+ 8x- 16=0
<=> x²+ 8x+ 16- 32=0
<=> (x+4)²= 32
TH1: x+ 4= √32 <=> x= -4+ √32
TH2: x+ 4= -√32 <=> x= -4- √32
Vậy x ∈ {-4+ √32; -4- √32}
16/ x² (3x + 1) - (x - 3)²= 3x³
<=> 3x³+ x²- (x²- 6x+9)- 3x³=0
<=> 3x³+ x²- x²+ 6x-9- 3x³=0
<=> 6x- 9 =0
<=> 6x=9
<=> x= 3/2
Vậy x= 3/2
