a) x2 – 4x + 3
= x2 – x – 3x + 3
= x(x – 1) – 3(x – 1)
= (x – 1)(x – 3)
b) x2 + 5x + 4
= x2 + x + 4x + 4
= x(x + 1) + 4(x + 1)
= (x + 1)(x + 4)
c) x2 – x – 6
= x2 – 3x + 2x – 6
= x(x – 3) + 2(x – 3)
= (x – 3)(x + 2)
d) x4 + 4
= x4 + 2.2.x2 + 4 – 4x2
= (x2 + 2)2 – (2x)2
= (x2 + 2 – 2x)(x2 + 2 + 2x)