Bài 1:
 a) 2x² – 4x
    = 2x*(x – 2)
 b) x² – 2x – 9y² +1
    = (x – 1)² – 9y²
    = (x – 1 + 9y)*(x – 1 – 9y)
 c) 5x³y – 10 x²y² + 5xy³
    = 5xy*(x² – 2xy + y²)
    = 5xy*(x – y)²
 d) 2x² + 7x – 15
    = 2x² +10x – 3x – 15
    =(2x² + 10x) – (3x + 15)
    = 2x*(x – 5) – 3*(x – 5)
 e) x³ – 2x² + x –xy²
    = x*(x² – 2x + 1 – y²)
    = x*[(x² – 2x + 1) – y²]
    = x*[(x – 1)² – y²]
    = x*(x – 1 + y)*(x – 1 – y)
 f) 4x² – 16x + 16
    = 4*(x² – 4x + 4)
    = 4*(x – 2)²
Bài 5:
 a) 1/2x*(x² – 4) = 0
<=> 1/2x = 0   hoặc  x² – 4 = 0
<=>   x = 0        hoặc (x + 2)*(x – 2) = 0
<=>   x = 0        hoặc x + 2 = 0 hoặc x – 2 = 0
<=>   x = 0        hoặc x = –2 hoặc x = 2
 b) (x + 2)² – (x – 2)*(x + 2) = 0
<=> (x + 2)*(x + 2 – x + 2) =0
<=> (x + 2)*4 = 0
<=> 4x + 8 = 0
<=> 4x = –8
<=> x = –8/6
Bài 8: 
 b) 8x² – 2
    = 2*(4x² – 1)
 c) x² – 6x – y² + 9
    = (x² – 6x + 9) – y²
    = (x – 3)² – y²
    = (x – 3 + y)*(x – 3 – y)