Question

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

Answer 1

The answer is

X^4 +2X^2 +3

Answer 2

(x-1)(x+1)=(x^2-1^2)

similarly

(x^2-3^2)

then

x^2×x^2 -x^2×3^2-1^2×x^2+1^2×3^2

=x^4-9x^2-x^2+9

=-9x^2+9

=9(x^2+1)