## Elementary and Intermediate Algebra: Concepts & Applications (6th Edition)

$16x^4-81$
Use the formula $(a+b)(a-b)$ to multiply the last 2 factors to obtain: $=(4x^2+9)[(2x)^2-3^2] \\=(4x^2+9)(4x^2-9)$ Use the same formula again with $a=4x^2$ and $b=9$ to obtain: $=(4x^2)^2-9^2 \\=16x^4-81$