Phân tích mỗi đa thức sau thành nhân tử.

a) 5x – 15y;

b) 12y(2x – 5y) + 6xy(5 – 2x);

c) x² – 7x + 12.

a) 5x – 15y = 5(x – 3y).

b) 12y(2x – 5y) + 6xy(5 – 2x)

= 6y[2.(2x – 5y) + x(5 – 2x)]

= 6y.(4x – 10y + 5x – 2x²)

= 6y(–2x² + 9x – 10y).

c) x² – 7x + 12

= x² – 4x – 3x + 12

= (x² – 4x) – (3x – 12)

= x(x – 4) – 3(x – 4)

= (x – 3)(x – 4).