Question

Which product will result in a sum or difference of cubes?

A (x + 7)(x2 – 7x + 14)
B (x + 8)(x2 + 8x + 64)
C (x – 9)(x2 + 9x + 81)
D (x – 10)(x2 – 10x + 100)

Answer 1

C. (x - 9)(x^2 + 9x + 81).

Explanation:

The cube identities are

a^3 - b^3 = (a - b)(a^2 + ab + b^2)

a^3 + b^3 = (a + b)(a^2 - ab + b^2)

Checking against the list the one that fits is the difference formula:

x^2 - 9^2 = (x - 9)(x^2 + 9x + 81).

a=1, b = 9, ab = 1 *9 = 9.