5x4 – 405

= 5(x4 – 81)

= 5(x2 + 9)(x2 – 9)

= 5(x2 + 9)(x + 3)(x – 3)by D. Fisherby D. Fisher

x2 + 10x + 16

= (x + 8)(x + 2)

x2 + 4x – 21

= (x + 7)(x – 3)

2x2 + 7x + 3

= (2x + 1)(x + 3)

3x2y + 12xy

= 3xy(x + 4)

x3 + 5x2 – x

= x(x2 + 5x – 1)

x2 – 81

= (x + 9)(x – 9)

4x2 – 121 =

(2x + 11)(2x – 11)

x2 – 14x + 49

= (x – 7)2

x2 + 8x + 16

= (x + 4)2

25x2 – 10x + 1

= (5x – 1)2

x2 – x – 6

= (x – 3)(x + 2)

3x2 +24x + 21

= 3(x + 1)(x + 7)

= 3(x2 + 8x + 7)

x4 – 16

(x2 + 4)(x – 2)(x + 2)

= (x2 + 4)(x2 – 4)

x2y +4xy + 4y

= y(x + 2)2

9x2 + 6x + 1

= (3x + 1)2

2x2 – 18x + 40

= 2(x – 4)(x – 5)