Question

How do you multiply (x - 1)(x + 1)(x - 3)(x + 3)?

Answer 1

`x^4 -10x^2 +9`

Explanation:

Frist of all, use twice the formula `(a+b)(a-b)=a^2-b^2` to obtain

`color(green)( (x-1)(x+1) ) color(blue) ( (x-3)(x+3) ) = color(green) ((x^2-1)) color(blue)((x^2-9))`

If you're satisfied, you can leave the expression like this. Otherwise, you can go for a full expansion, multiplying term by term, and obtaining

`(x^2-1)(x^2-9) = x^4 -9x^2 -x^2 +9 = x^4 -10x^2 +9`