c) xy – 5y + 4x – 20;
c) xy – 5y + 4x – 20;
c) Cách 1:
xy – 5y + 4x – 20
= (xy – 5y) + (4x – 20)
= y(x – 5) + 4(x – 5)
= (x – 5)(y + 4).
Cách 2:
xy – 5y + 4x – 20
= (xy + 4x) ‒ (5y + 20)
= x(y + 4) ‒ 5(y + 4)
= (y + 4)(x ‒ 5).