b) `x^2 - 4 = x^2 - 2^2 = (x-2)(x+2)`

c) `1 - 8x^3 = 1 - (2x)^3 = (1 - 2x) (1 + 2x + 4x^2)`

d) `x^2 + 6x + 9 = x^2 + 2 . 3 . x + 3^2 = (x + 3)^2`

e) `10x - 25 - x^2 = - x^2 + 10x - 25 = - (x^2 - 10x + 25) = - (x^2 - 2 . 5 . x + 5^2) = - (x - 5)^2`

f) `8x^3 - 1/8 = (2x)^3 - (1/2)^3 = (2x - 1/2) (2x + x + 1/4)`

g) `1/25 x^2 - 64 y^2 = (1/5x)^2 - (8y)^2 = (1/5x - 8y)(1/5x + 8y)`

h) `x^3 + 1/27 = x^3 + (1/3)^3 = (x + 1/3) (x^2 - 1/3x + 1/9)`

k) `(a + b)^3 - (a - b)^3 = (a + b - a + b) [(a+b)^2 + (a + b)(a-b) + (a-b)^2] = 2b . [a^2 + 2ab + b^2 + a^2 - b^2 + a^2 - 2ab + b^2] = 2b [3a^2 + b^2]`

l) `(a + b)^3 + (a - b)^3 = (a + b + a - b) [(a+b)^2 - (a + b)(a-b) + (a-b)^2] = 2b . [a^2 + 2ab + b^2 - a^2 + b^2 + a^2 - 2ab + b^2] = 2a [a^2 + 3b^2]`